Allow label descriptions to be manipulated

[jamesodhunt]

This is a great tool! I think it would be even better if it was possible to add/change/delete the label descriptions.

fwics, the vendoring for github.com/google/go-github needs to be updated as the currently vendored version doesn't include Description in the Label definition:

• https://github.com/google/go-github/blob/master/github/issues_labels.go#L19

[jamesodhunt]

Ah - I've just seen that #37 has already been raised to add description support! 😄