Add support for GH enterprise

[xEtherealx]

I'm guessing the below failure is due to lack of support for self-hosted gh instances; it would be great to get support here but if there's a workaround please lmk!

❯ spr init
  👋  Welcome to spr!

  🔑  Okay, lets get started. First we need a 'Personal Access Token' from GitHub. This will authorise
      spr to open/update/merge Pull Requests etc. on behalf of your GitHub user.
      You can get one by going to https://github.com/settings/tokens and clicking on 'Generate new
      token'. The token needs the 'repo', 'user' and 'read:org' permissions, so please tick those two
      boxes in the 'Select scopes' section.
      You might want to set the 'Expiration' to 'No expiration', as otherwise you will have to repeat
      this procedure soon. Even if the token does not expire, you can always revoke it in case you fear
      someone got hold of it.
      Please paste in your PAT and press enter. (The input will not be displayed.)
GitHub Personal Access Token: [hidden]
  🛑  GitHub: Bad credentials
      Documentation URL: https://docs.github.com/rest

[kaczordon]

+1 it would be great to be able to specify or override the gh instance spr is talking to. But a workaround would be great too as this tool is super exciting!

[andrewhamon]

Would like to +1 this, wanted to check this out but we use GitHub enterprise on prem as well.

[andrewhamon]

Submitted a PR for GHE support in #158