MDLSITE-3538 commit checker: disable some merge-commit checks

We don't want to enforce any branch naming schema, so merge
commits don't need to match the MDL issue anymore, as was
planned originally @ MDLSITE-1990.

So putting that under the control of a new, disabled by default,
new parameter.
Merge pull request #22 from stronk7/MDLSITE-3538

MDLSITE-3538 commit checker: disable some merge-commit checks