Implement the tags feature

[jiegillet]

Please read exercism/elm#632 first, this issue focuses on implementation.

This issue is a roadmap of things left to do before we can ship the tags feature:

• Implement the framework for extracting tags using elm-review --extract (Proposal for tagging mechanism #79)
• Implement all tags listed in this spreadsheet. This can be done in small chunks. New tags can be freely added there, if you want to remove tags, we can discuss it on this issue for better visibility, or in PRs
• Add mechanism for ignoring editor and exemploid files. Either using the config file (preferred) or a naming convention
• Add better smoke tests. In particular, check that editor and test files are ignored, and also for the most complex tags (recursion...)
• Remove --tags feature flag. This is the last step and will make the feature go live