YAML file documenting Limited formats

[I started it here](https://github.com/taw/magic-search-engine/blob/master/data/formats.yaml)

TODO:

- [ ] figure out how that format would work, especially prerelease formats have quite a few special cases like faction choices, playable promos etc.
- [ ] get the data
- [ ] consume this on the website
- [ ] export it for mtgjson