Both rbt post and rbt publish can now publish with a change

description. This change description supports markdown via the

--markdown flag used for summaries and descriptions.
Providing a change description before the review request is public will

not result in an error; instead the option will be ignored and an

informative message will be displayed to the user.
The publish command now also only fetches the required links and

fields when publishing to minimize the amount of data that is

transmitted.

Published with a message.
Verified that rbt publish -m provided an error message when the

  review request was not public.