I'm trying to create a nightwatchJS custom assertion that will check if a file exists. The assertion appears to fire, but nightwatch exits as soon as the assertion command finishes.

I guess I'm not returning control to the nightwatch api, but if thats the case then how can I achieve that?

```js
  // filename = doesFileExist.js

  exports.assertion = function(fname, msg) {
   var fs = require('fs');

   this.message = msg + 'FileExists: ' + fname ;     
   this.expected = true;                     
   this.pass = function(value) {             
       return value == this.expected;
        } ;

   this.value = function(result) { 
       return result;       
       };    

   this.command = function(callback) { 
      return fs.exists(fname, callback);
       };
};
```

and the test case (using nightwatch.json as an example) is ;

```js
  this.checkForFile = function() {
    browser
      .verify.doesFileExist('nightwatch.json', 'test1')  
    return browser;
  };
```