search-tags: Add support for search-terms.

This patch adds support for search-terms to 'notmuch search-tags'. If
no search-term is provided then the command returns a list of all tags
from the database.

If the user provides one or more search-terms as arguments then the
command collects tags from matching messages only.

This could be used by functions in the Emacs mode to further limit the
list of tags offered for completion. For example, functions that remove
tags from message(s) could offer only tags present in the message(s).

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