notmuch.el: Select tag names with completion.

Several commands ask the user for a tag name. With this feature the
user can just press tab and Emacs automatically retrieves the list of
all existing tags from notmuch database with 'notmuch search-tags' and
presents a completion buffer to the user.

This feature is very useful for users who have a large number of tags
because it saves typing and minimizes the risk of typos.

Signed-off-by: Jan Janak <jan@ryngle.com>