Support Github Enterprise Server

[rodrigost23]

Is your feature request related to a problem? Please describe.
A clear and concise description of what the problem is.
I cannot use this extensions with GHES using a custom domain

Describe the solution you'd like
A clear and concise description of what you want to happen.
To be able to enable domains other than github.com, such as refined-github/refined-github#450

[NirmalScaria]

I wont be able to work on this because I don't have access to an enterprise server to test this.
The issue will be kept open for anyone who would like to give it a try.