Add 'cat' subcommand

Add 'cat' subcommand

This command outputs a raw message matched by search term to the
standard output. It allows MUAs to access the messages for piping,
attachment manipulation, etc. by running notmuch cat rather then
directly access the file. This will simplify the MUAs when they need
to operate on a remote database.

Edited-by: Carl Worth <cworth@cworth.org>: Remove trailing whitespace,
add missing "test_done" to new test script to avoid "Unexpected exit"
error.