support a "selection search"?

[ctb]

right now, mgmanysearch searches all queries against all metagenomes.

but, in general, you only want to do specific pairs.

seems like it might be worthwhile to support passing in the set of pairs we care about. not quite a picklist, but similar.

since mgmanysearch optimizes for loading metagenomes, this might help us optimize for that case by doing all the queries for each loaded metagenome.

Related issues:

• consider supporting picklist-style usage of output from manysearch and mastiff => optimization #12

[ctb]

cc @AnneliektH - seem valuable?