Markus Koller authored Jul 09, 2019 and Achilleas Pipinellis committed Jul 09, 2019

The configuration is currently only specified in CI, by moving it into
`.mdlrc` we get immediate feedback locally in supported editors.

To ensure `mdl` is available it's also added to the `Gemfile`, though
CI will still use the version installed in the `gitlab-docs-lint` image.