GitHub Enterprise Integration bug

[cyb3rko]

Hey there, first of all thanks for the library.

I've tried using it with a GitHub Enterprise instance, unfortunately it's not fully working yet:

It seems that the images could not be fetched correctly and the hyperlinks of the contributors link to
https://github.com/<user>
instead of
https://<enterprise_domain>/<user>

I suppose this line of code is the problem: https://github.com/byrnereese/mkdocs-git-committers-plugin/blob/5135dea4b9672a932f9718784c86b292768e6cd4/mkdocs_git_committers_plugin/plugin.py#L65