notmuch: Support "notmuch part" as an alias for "notmuch show --format=raw"

We unifed the "notmuch part" functionality into "notmuch show" where
the implementation is both simpler and more powerful. But there's no
good reason to break users of the old interface.

Add support for aliases, which are undocumented means of getting at
functionality through deprecated names. The first such alias is
"notmuch part" as implemented here.