emacs: allow notmuch-tag to accept string inputs and prompt for tags

notmuch-tag is extended to accept various formats of the tag changes.
In particular, user prompting for tag changes is now incorporated
here, so it is common for modes.

The tag binary and the notmuch-{before,after}-tag-hooks are only
called if tag changes is non-nil.

In all cases tag-changes is returned as a list.