Add support for GitHub Enterprise domains #315

<table><th>❇️ <tr><td> @fregante wrote: Hey Google user! If you’re looking for a way to add GitHub Enterprise support to your browser extension, check out this guide:

https://github.com/fregante/webext-dynamic-content-scripts/blob/master/how-to-add-github-enterprise-support-to-web-extensions.md

I added this message here because I noticed that this issue is one of the first results.

<tr><th> ❇️ </table>

I think it's great that you've limited the extension to only accessing data on the github.com domain but this prevents me from using it with GitHub Enterprise which runs on an internal domain.