lei: per-message keywords and externals

[Eric Wong <e@80x24.org>]

Something I've been pondering for a bit is how to handle
keywords (Seen, Important, Replied, ...) for messages stored in
externals.

I want "kw:" prefix to be a usable search term, like:

	lei q something interesting kw:seen
	lei q something interesting NOT kw:seen

This is no problem for imported messages in ~/.local/share/lei/store.
All the keyword info is stored in line with the rest of the
Xapian index data.

But, I also don't want to be wasting users' space by duplicating
index data if they're already hosting inboxes for public
consumption.  So, it's looking like parsing out kw: ourselves
and do extra filtering on our end when externals are in play is
going to be a requirement...

Or, just don't support searching using "kw:" with externals, for
now; but still stash keywords somewhere when writing to
traditional mail stores.

And there's also HTTP/HTTPS externals, but those will have
transparent caching/memoization into lei/store by default, soon.