Add 'cat' subcommand

Add 'cat' subcommand

This command dumps raw message identified by filename to standard
output. It uses mail store interface to get the message from the right
place.

notmuch.el was modified to use this command to access the raw message
content (view/save attachments, view raw message and pipe message to
command).

With this patch, it is straightforward to use notmuch emacs interface
remotely over SSH. To do this, it is sufficient to redefine
notmuch-command variable to contain the name of a script containing:

    ssh user@host notmuch "$@"

Signed-off-by: Michal Sojka <sojkam1@fel.cvut.cz>