Install zsh completion file

According to the Debian zsh maintainer Clint Adams, this is the first
time that a package installs its own completer into zsh. Part of the
reason this is not usually done is because zsh does not provide a stable
API.

We agreed to try it, given that notmuch is expected to change quite
a bit initially. If there are problems or the completer goes stable,
we'll move it into the upstream zsh repository.

Signed-off-by: martin f. krafft <madduck@debian.org>