notmuch.el: Add commands to add tag, remove tag, and archive (== remove inbox tag)

These have keybindings of '+', '-', and 'a'. The bug they have so
far is lack of visual feedback for their effect, and lack of undo.

(Also the fact that adding or removing a single tag for a thread
takes way too long--but that's as a Xapian issue as discussed here:

	replace_document should make minimal changes to database file
	http://trac.xapian.org/ticket/250
)