Support github enterprise

Issue

Currently, this usefully package only works with default base_url for public github instances.

Be able to copy labels between projects of github enterprise instances.

Add an optional parameter "base_url" to pass in the CLI, in order to call Github(token, base_url="https://my_company/api/v3") inside the lib.

PS : I see this repo has no activity for a while, so if somebody can approve a PR and make a release, I can code this change