Issue title: Using highcharts theme
 Issue body: Hi,

I use your great application as node module and have couple of questions.

It is possible to use highcharts theme like dark-unica for my charts?

How can I exit from node script after done all tasks?

Thanks and keep up the good work
 Comments: 
 Comment 0: Hi,

To use dark-unica, you can add `http://code.highcharts.com/themes/dark-unica.js` to `resources.files`:
    
        {
            "files": "http://code.highcharts.com/themes/dark-unica.js"
        }

This works both when using the CLI and when doing HTTP requests. 

If through the HTTP server, your payload should look something like this:
    
        {
            "instr": "<chart data here>",
            "resources": {
                "files": "http://code.highcharts.com/themes/dark-unica.js"
            }
        }

If using the CLI:

        highcharts-export-server --resources '{"files": "http://code.highcharts.com/themes/dark-unica.js"}' --instr "<chart data>"

Regarding the exiting - are you using `highcharts-export-server` directly, or are you using it as a module? In the first case it should exit by itself when done, so if it doesn't, please send me a sample input, and I'll look into it.



 Comment 1: @cvasseng Please note that the theme files also load custom font files, so unless these are installed on the server instance, they also need to be considered. https://github.com/highcharts/highcharts/blob/v5.0.6/js/themes/dark-unica.js#L14-L18.
 Comment 2: @TorsteinHonsi since the font is included in a `<link>` tag that should work out of the box (famous last words). There seems to be an issue with the theme loading itself though, so I'm looking into that now.
 Comment 3: > that should work out of the box

Yes if it is loaded in sync it should work. 
 Comment 4: So turns out google fonts are "special" - the way they are loaded causes Phantom not to wait for them before executing the `onLoadFinished` callback. The issue is in the font css, where there's another indirect include - this is the part it doesn't wait for. Working on a solution.
 Comment 5: What I'm doing wrong? I tried any combination but theme is not applied.
Here example with added parameter the resources:
```
//Include the exporter module
const exporter = require('highcharts-export-server');

//Export settings
var exportSettings = {
  type: 'pdf',
  instr: {
    title: {
      text: 'My Chart'
    },
    xAxis: {
      categories: ["Jan", "Feb", "Mar", "Apr", "Mar", "Jun", "Jul", "Aug", "Sep", "Oct", "Nov", "Dec"]
    },
    series: [
      {
        type: 'line',
        data: [1, 3, 2, 4]
      },
      {
        type: 'line',
        data: [5, 3, 4, 2]
      }
    ]
  },
  resources: {
    files: 'http://code.highcharts.com/themes/dark-unica.js',
  },
  outfile: 'test.pdf',
};

//Set up a pool of PhantomJS workers
exporter.initPool();
/*
 Export settings corresponds to the available CLI arguments described
 above.
 */
exporter.export(exportSettings, function (err, res) {
  exporter.killPool();
});
```

Also I try:

```
resources: {
   files: './node_modules/highcharts/themes/dark-unica.js',
},
```
 And save to file 'resources.json' with:
```
{
  "files": "http://code.highcharts.com/themes/dark-unica.js"
}
```

BUT! When I set parameter the themeOptions like JSON.stringify({ // some theme options }); I get exactly what I want.
What I'm doing wrong? I tied any combination but theme is not applied.
Here example with added parameter the resources:
```
//Include the exporter module
const exporter = require('highcharts-export-server');

//Export settings
var exportSettings = {
  type: 'pdf',
  instr: {
    title: {
      text: 'My Chart'
    },
    xAxis: {
      categories: ["Jan", "Feb", "Mar", "Apr", "Mar", "Jun", "Jul", "Aug", "Sep", "Oct", "Nov", "Dec"]
    },
    series: [
      {
        type: 'line',
        data: [1, 3, 2, 4]
      },
      {
        type: 'line',
        data: [5, 3, 4, 2]
      }
    ]
  },
  resources: {
    files: 'http://code.highcharts.com/themes/dark-unica.js',
  },
  outfile: 'test.pdf',
};

//Set up a pool of PhantomJS workers
exporter.initPool();
/*
 Export settings corresponds to the available CLI arguments described
 above.
 */
exporter.export(exportSettings, function (err, res) {
  exporter.killPool();
});
```

Also I try:

```
resources: {
   files: './node_modules/highcharts/themes/dark-unica.js',
},
```
 And save to file 'resources.json' with:
```
{
  "files": "http://code.highcharts.com/themes/dark-unica.js"
}
```

BUT! When I set parameter the themeOptions like JSON.stringify({ // some theme options }); I get exactly what I want.
What I'm doing wrong? I tied any combination but theme is not applied.
Here example with added parameter the resources:
```
//Include the exporter module
const exporter = require('highcharts-export-server');

//Export settings
var exportSettings = {
  type: 'pdf',
  instr: {
    title: {
      text: 'My Chart'
    },
    xAxis: {
      categories: ["Jan", "Feb", "Mar", "Apr", "Mar", "Jun", "Jul", "Aug", "Sep", "Oct", "Nov", "Dec"]
    },
    series: [
      {
        type: 'line',
        data: [1, 3, 2, 4]
      },
      {
        type: 'line',
        data: [5, 3, 4, 2]
      }
    ]
  },
  resources: {
    files: 'http://code.highcharts.com/themes/dark-unica.js',
  },
  outfile: 'test.pdf',
};

//Set up a pool of PhantomJS workers
exporter.initPool();
/*
 Export settings corresponds to the available CLI arguments described
 above.
 */
exporter.export(exportSettings, function (err, res) {
  exporter.killPool();
});
```

Also I try:

```
resources: {
   files: './node_modules/highcharts/themes/dark-unica.js',
},
```
 And save to file 'resources.json' with:
```
{
  "files": "http://code.highcharts.com/themes/dark-unica.js"
}
```
BUT! When I set parameter the themeOptions like JSON.stringify({ // some theme options }); I get exactly what I want.
 Comment 6: I am having the issue related the the google font not loading in time for the graph to use it.
If i hit the same phantom worker a 2nd time the font loads (probably cached from previous request)
 Comment 7: Finally got to the bottom of this.

a5ad761 fixes font issues with the Dark Unica and Sand Signika themes. Including them in resources should now work as expected.

The issue with google fonts is that Phantom/WebKit won't actually load the fonts unless there's a reference to it in the DOM. This is problematic both in the sense that the required font won't be loaded, but even more so because the reference to the included font is in place regardless, causing it to think that the font is loaded too. The result is that all text is gone. 

Working on finding a more robust/clean/general solution, the progress of which can be tracked in #13.


{'source': 'GithubIssues', 'repo_name_with_owner': 'highcharts/node-export-server', 'issue_id': 194253091, 'repo_stars': 330.0}