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.
Tagging functions are used in notmuch.el, notmuch-show.el, and
notmuch-message.el. There are enough common functions for tagging
that it makes sense to put them all in their own library.
No code is modified, just moved around.
