Add notmuch-folder mode to provide an overview of search matches

Add notmuch-folder mode to provide an overview of search matches

Folder mode takes a (user-configurable) list of search patterns and
produces a list of those patterns and the count of messages that they
match. When an entry in this list is selected, a search window with
the defined search is opened.  The set of folders is defined as a
list, each element contains the name of the folder and the query string
to count.

Signed-off-by: Keith Packard <keithp@keithp.com>