editLabel method
Edits a label.
API docs: https://developer.github.com/v3/issues/labels/#update-a-label
Implementation
Future<IssueLabel> editLabel(RepositorySlug slug, String name, String color) {
return github.postJSON('/repos/${slug.fullName}/labels/$name',
body: GitHubJson.encode({'name': name, 'color': color}),
convert: (dynamic i) => IssueLabel.fromJson(i));
}