Add support for .gitlab-ci.yaml files

[ilovelinux]

Fixes #595

[sirosen]

Thanks for filing the issue and PR! I had been under the impression that GitLab only supports the .yml suffix -- that's what's in their docs, and they do not mention support for .gitlab-ci.yaml.

Can you confirm that GitLab supports the .yaml suffix and will execute pipelines from configs with this extension?

There are a couple of things which need to be tweaked for this to merge:

• CI tests confirm that .yaml is rejected by the regex -- that needs to be updated
• per the comment in the .pre-commit-hooks.yaml file, that hook is autogenerated, so the config which generates it needs to be updated