Go-github: Support new Repository Transfer API

[gmlewis]

GitHub Developers API announcement:
https://developer.github.com/changes/2017-11-09-repository-transfer-api-preview/

GitHub v3 API developer docs:
https://developer.github.com/v3/repos/#transfer-a-repository

This would be a great project for a new contributor to this repo!
The new endpoint should fit nicely into one of the existing source files, possibly repos.go unless a better alternative is found and we can discuss it.

All contributors and contributions are greatly appreciated. Thank you for your help in keeping this GitHub API client library for Go up-to-date!

[juliaferraioli]

This sounds like a perfect issue for me to get involved on. Sign me up!

[sahildua2305]

Indeed, I think this will fit well in repos.go. Welcome @juliaferraioli! 🎉