Issue title: json format
 Issue body: Very nice that one can specify the json file.  However, what if the format is not just an array of objects?  Is this supported?  For example:

{
  "docs": [
    {"Movie_name": "Goodfellas","Movie_year": 1990},
    {"Movie_name": "Meet the Fockers","Movie_year": 2004},
    {"Movie_name": "Shark Tale","Movie_year": 2004},
    {"Movie_name": "Raging Bull","Movie_year": 1980},
    {"Movie_name": "Untouchables, The","Movie_year": 1987},
    {"Movie_name": "Godfather: Part II, The","Movie_year": 1974},
    {"Movie_name": "Meet the Parents","Movie_year": 2000},
    {"Movie_name": "Deer Hunter, The", "Movie_year": 1978}
  ]
}

 Comments: 
 Comment 0: Of course yes, you can use:
- `responseHandler` option, for example:
  http://wenzhixin.net.cn/p/bootstrap-table/docs/examples.html#card-view
  http://wenzhixin.net.cn/p/bootstrap-table/docs/data4.json
- `load` method (via javascript):

```
$.get('data.json', function(res) {
    $('#table').bootstrapTable('load', res.docs);
});
```

{'source': 'GithubIssues', 'repo_name_with_owner': 'wenzhixin/bootstrap-table', 'issue_id': 38212454, 'repo_stars': 11395.0}