Added extra_config properties directly in config.yml

In our projects involving Ckeditor and the TrsteelCkeditorBundle, we need sometime to add some plugins that require their own configuration. 
This configuration sometine implies the use of Symfony2 routes, that we can't just add in a hardcoded way in some custom config.js file.

In order to be able to configure different types of custom configurations, I modified the bundle to allow the user to set either scalar, file, or route names parameters. The old way to configure custom.js file is still valid.

For example:
```
trsteel_ckeditor:
    ...
    # This is valid
    custom_config:
        filebrowserVideoBrowseUrl:
            type: route
            value: elfinder

    # This is also valid
    custom_config: someConfig.js
```

What do you think about this feature ?