emacs: Make the queries used in the all-tags section configurable

This patch adds a customization variable that controls what queries
are used to construct the all-tags section in notmuch-hello. It allows
the user to specify a function to construct the query given a tag or
a string that is used as a filter for each tag.
It also adds a variable to hide various tags from the all-tags section.

Signed-off-by: Daniel Schoepe <daniel.schoepe@googlemail.com>