Rename "notmuch cat" to "notmuch show --format=raw"

This is part of an effort to avoid proliferation of excessive
top-level notmuch commands. Also, "raw" better captures the
functionality here, (as opposed to "cat" which is a fairly oblique
reference to a bad Unix abbreviation whose metaphor doesn't work here
since "notmuch cat" operates only on a single message and hence cannot
"con'cat'enate" anything).