Allow combining an RTree query with a secondary index

[sbooth]

This is an enhancement stemming from #295

To quote @robbiehanson:

It may be ideal for us to figure out a way to combine an RTree query with a secondary index. In this example, he could easily setup a secondary index on 'Trace.type'. Then combining the RTree query and the secondary index into a single SQLite query under-the-hood would yield an optimal query.

The question for us is: "How can we improve the API to make this easy/possible to do?"