second round of indexing all files

[David Bremner]

This adds in a "notmuch reindex" command so that deleting the terms
from deleted files can be accomplished.  There are still several UI
issues to deal with (i.e. we return an arbitrary file, not necessarily
the one matched).

The reindex command is a simplified version of one the that dkg
originally wrote for his series on indexing encrypted messages. I've
ripped out all the encryption related stuff here.

I've also postulated (but not yet written) a more generic way of
handling index options, roughly modeled on our command-line-options
code. I hope that this will allow fewer functions, and a more static
API at the library level; at this point it's just a sketch of an idea.