Ability to customize columns used by extracts= feature

@simonw any thoughts on allow extracts to specify the lookup column name? If I'm understanding the documentation right, .lookup() allows you to define the "value" column (the documentation uses name), but when you use extracts keyword as part of .insert(), .upsert() etc. the lookup must be done against a column named "value". I have an existing lookup table that I've populated with columns "id" and "name" as opposed to "id" and "value", and seems I can't use extracts=, unless I'm missing something...

Initial thought on how to do this would be to allow the dictionary value to be a tuple of table name column pair... so:

table = db.table("trees", extracts={"species_id": ("Species", "name"})

I haven't dug too much into the existing code yet, but does this make sense? Worth doing?

Originally posted by @chrishas35 in https://github.com/simonw/sqlite-utils/issues/46#issuecomment-592999503

Start by reading how .lookup() defines its value column and how the extracts keyword is handled by .insert() and .upsert(). Confirm the supported API for specifying a lookup table and column, then add coverage showing an existing table using id and name works with extracts; the issue does not mention specific files or tests.