backports: Allow setting a custom priority.

The module used to always pin backports to a priority of 200. This
default is still retained but is now configurable.

Additionally the default is now an Integer, not a 'quoted Integer' and
the tests have been updated to reflect this. This matters for future
parser as it will now kick people if they pass in a stringified integer
as priority.
Merge pull request #275 from daenney/backports-pin-priority

backports: Allow setting a custom priority.