Add specific feature requests to API

[gliese1337]

This should be another backwards-compatible addition.

Currently, we return the first results we get from any service, even if those results are not very complete. We should be able to take requests for specific kinds of data from the client (like "make sure we get full translations" or "make sure the part-of-speech is included") and then continue making requests, combining lemmas where possible, until at least one result is available that satisfies all of the requests.

In the event that all requests cannot be satisfied, we'd fall back to just returning the best, most complete result that is available.