By allowing the user to specify the generator_id it is possible to use any [register generator](http://django-imagekit.readthedocs.org/en/latest/#using-specs-in-templates).

This makes it easy to configure, the only limitation would be the limited set of `kwargs` - we could extend it to pass all `QUERY_PARAMS` to generator.

@meshy do you think this is wise? It would be possible to send a combination of params and generator that do not work, maybe this is ok as long as it returns an appropriate error.