Support a context option to be posted back by browse-everything

Allows a `data-context` option to be added to the trigger button (or a `context` key added to the options passed to `$().browseEverything()`) that will be passed back to the server whenever the user navigates to a new provider or folder, or when the submit button is clicked. This will allow an application-supplied `before_filter` to alter the server-side, request-specific browse configuration before the request is handled (e.g., to allow the file_system handler to point to a different location).

```html
<button class="btn btn-large btn-success" data-context="my-context" 
  data-route="/browse/" data-target="#main_form" 
  data-toggle="browse-everything" id="browse-btn" 
  name="button" type="button">Browse</button>
```
Totally pedantic but what's the purpose of this variable?  It seems like `file_type` is only used once below, and isn't particularly conveying any information that isn't in the method name on the object (`file.type`).