Support filtering records to index based on schema version

[thatbudakguy]

related to #105.

for folks not using aardvark, we want to support the possibility of harvesting scoped to earlier schema versions. the "simple" way to do this is just e.g. looking for /aardvark/ in the path and asking folks to put their records in an "aardvark" folder, but ideally we would do it the "smart" way and actually look at the schema version in the record at the point we see the record. if this isn't possible for performance reasons, we can fall back to the "simple" strategy.