Summary:
Add API for #315. Not sure if this is what you want rnystrom though 🤔. Will add tests after you confirm.
- [x] All tests pass. Demo project builds and runs.
- [x] I added tests, an experiment, or detailed why my change isn't tested.
- [ ] 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/456
Reviewed By: jessesquires
Differential Revision: D4697190
Pulled By: rnystrom
fbshipit-source-id: f8513cf2fa33441eb40f486954136553d19dda0c