This feature updates rbt post to allow for setting custom fields when 

creating/updating a review request. It adds a new --field key=value

arguments, which would set extra_data.key=value in the payload. It 

currently handles the native fields (description, testing_done, 

summary) turning them into aliases of --description, --testing-done, 

--summary. It also allows for multiple field arguments to be set

added --field description="desc" to an rbt post
Ensured that the field description contained "desc"
Added multiple fields --field description="high level desc"

--field summary="short summary" to an rbt post
Ensured both argument fields were filled in correctly
Added Unit tests