Load custom plugins & rules automatically

What does this MR do and why?

This is a major improvement where custom plugins and rules are automatically loaded by the gem.

All you have to do is to define gitlab_dangerfiles.config.ci_only_rules in the Gitlab::Dangerfiles.for_project block if needed.

This also brings a Gitlab::Dangerfiles.load_tasks method (to be called in your Rakefile) to add the danger_local task to your project.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.