Summary:
Adds the source for a remodel plugin to autogenerate models conforming to `IGListDIffable`.
- [x] All tests pass. Demo project builds and runs. (source untouched, purely additive)
- [x] I added remodel plugin tests + and a new guide explaining the installation + usage
- [ ] I added an entry to the `CHANGELOG.md` for any breaking changes, enhancements, or bug fixes.
- [x] I have reviewed the [contributing guide](https://github.com/Instagram/IGListKit/blob/master/.github/CONTRIBUTING.md)
Closes https://github.com/Instagram/IGListKit/pull/1083
Reviewed By: rnystrom
Differential Revision: D6937925
Pulled By: calimarkus
fbshipit-source-id: 54228cddf387cf20ceceec1732e439ab6fd4b984
