List does not allow filteration by label

[kashifrazzaqui]

This is very useful and provided by the underlying api and I would like to use it for my cmd line tool - https://github.com/kashifrazzaqui/github-issues

If you will please add this.
Thanks.

[JNRowe]

A list_by_label function, implemented by @bartdag, was merged earlier today. A new release will hit PyPI in the next day or two.

Thanks