emacs: Access raw messages via cat subcommand

This patch modifies the following commands to access the messages via
cat subcommand:
- view/save attachments ('v', 'w'),
- view a raw message ('V') and
- pipe a message to a command ('|').

With this patch, it is straightforward to use notmuch emacs interface
with a remote database accessed 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 "$@"

If the ssh client has enabled connection sharing (ControlMaster option
in OpenSSH), the emacs interface is almost as responsive as when
notmuch is invoked locally.