create-pull-request: provide an RFC mode via -c argument

create-pull-request: provide an RFC mode via -c argument

Currently it is difficult to know if a pull request is being sent for review
or just to be pulled.

Add a -c argument to add RFC to the subject prefix and a blurb requesting
review to the cover letter.

Signed-off-by: Darren Hart <dvhart@linux.intel.com>
Acked-by: Joshua Lock <josh@linux.intel.com>
Acked-by: Otavio Salvador <otavio@ossystems.com.br>
Cc: Richard Purdie <richard.purdie@linuxfoundation.org>
Cc: Saul Wold <sgw@linux.intel.com>
Cc: Paul Eggleton <paul.eggleton@intel.com>
Cc: Joshua Lock <josh@linux.intel.com>
Cc: Otavio Salvador <otavio@ossystems.com.br>