Add support for private repositories to `from_github()`

Prework

• Read and abide by pointblank's code of conduct and contributing guidelines.
• Search for duplicates among the existing issues (both open and closed).

Proposal

from_github gives the user a way to retrieve data from github. Example scenarios where this maybe be useful are version controlled schema definitions against which table structure can be compared. However, this function currently only supports public repositories.

I am based on a private organisation and so I cannot use from_github. It would be valuable to add support for private repositories as well. I am limited of time but am willing to look into this at some point.