Add support for label description field #400

Hi 👋,

Firstly, thank you for creating this beautiful package. I am creating a Github App that heavily relies on labels, and would need access to the description field of labels in the repository. Would it be possible that you add support for that? If you'd guide me, I could also create a PR.

Running through the source code, I believe these are the files that'd need to be changed:

• https://github.com/phadej/github/blob/master/src/GitHub/Endpoints/Issues/Labels.hs
• https://github.com/phadej/github/blob/master/src/GitHub/Data/Definitions.hs#L262

https://developer.github.com/v3/issues/labels/#create-a-label for the reference