lint config files

This adds support for linting config files like config.yml, config/releaseChannels.yml, etc. This is a new concept here since fdroid lint has not really ever handled config files. With localization, mirror metadata, and more there is a lot more stuff that people are editing in the config. Then these are also maintained in fdroiddata and can be edited via merge requests, so there needs to be CI checks.

@jspricke @FestplattenSchnitzel @linsui @licaon-kter @uniqx this is conceptually new in fdroid lint so I want to make sure everything things is make sense.

Also, this switches loading config/mirrors.yml with ruamel to get YAML 1.2 support.