emacs: add notmuch-before- and notmuch-after-tag-hook

This patch adds hooks that are run before/after messages are tagged
From the emacs interface.  In order to implement this and to avoid
having hooks parse all the arguments to the notmuch binary again, I
created a `notmuch-tag' function that other modules should use instead
of running (notmuch-call-notmuch-process "tag" ...) directly.