lib: New function to collect tags from a list of messages.

This patch adds a new function that can be used to collect a list of
unique tags from a list of messages. 'notmuch search-tags' uses the
function to get a list of tags from messages matching a search-term,
but it has the potential to be used elsewhere so we put it in the lib.

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