Allow adding description to label

[iforwms]

API supports adding description to label, see below:

[gregoralbrecht]

Seems like this should be tackled in git-label and someone already opened a PR a while ago.

[Yamilquery]

+1

[iforwms]

@gregoralbrecht I created a very naive implementation written in PHP, and the above pull request won't work (I don't think). I think it's still an experimental feature, so the you will also need to add the following header to the request or descriptions will be ignored.

'Accept' => 'application/vnd.github.symmetra-preview+json'