This adds completion files for zsh that cover most of notmuch's cli.
The files in completion/zsh are formatted so that they can be found by
zsh's completion system if put $fpath. They are also registered to the
notmuch-* pattern, so they can be called externally using _dispatch.
Update installation recipe and drop debian/notmuch.examples to avoid
breakage. This means zsh completion is not installed for debian, to be
fixed in a future commit.
Amended by db: use regexp searching for address completion. This seems
to be fast enough to work without a cache.
