Support for indexing fields of related models? #6

Hey, awesome library! I was disappointed to see that Django does not offer any support out of the box for FTS for real applications - using triggers and store procedures instead of indexing on the fly - which obviously does not scale on even a modest level of traffic.

For our use case, we'd like to index fields on related models through joins. One simple example to allude to is the ability to search for orders by line item descriptions. Briefly digging into the codebase, it does not look like this is currently supported.

I'd be happy to contribute something to make this work. Has this feature been thought about before? Any recommendations for how this should be tackled?