Add support for label description field

[maticzav]

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

[phadej]

Thank you for your kind words.

PR are always welcome.

[sajidanower23]

@maticzav If you haven't started working on it, I am happy to look into it this week :)

[sajidanower23]

@maticzav It appears that @phadej is on a break. If it's urgent you can use #418 for the moment 👍