emacs: User-defined sections in notmuch-hello

emacs: User-defined sections in notmuch-hello

This patch makes the notmuch-hello screen fully customizable
by allowing the user to add and remove arbitrary sections. It
also provides some convenience functions for constructing sections,
e.g. showing the unread message count for each tag.

This is done by specifying a list of functions that will be run
when notmuch-hello is invoked.