Call me collections

- [x] Use Collections for `data` feature
- [x] Allow us to read in collections
- [x] Render collections
- [x] TomDoc the sh*t outta this code
- [x] Write docs
- [x] Expose to Liquid via `site.*`

/cc @gjtorikian

Fixes https://github.com/jekyll/jekyll/issues/1941.