emacs: address completion, allow sender/recipient and filters

This commit lets the user customize the address completion. It makes
two changes.

The first change controls whether to build the address completion list
based on messages you have sent or you have received (the latter is
much faster).

The second change add a possible filter query to limit the messages
used -- for example, setting this to date:1y..  would limit the
address completions to addresses used in the last year. This speeds up
the address harvest and may also make the search less cluttered as old
addresses may well no longer be valid.