Add support for GitHub Enterprise

[motionsuggests]

Describe Request:

We would like to use this orb with our GitHub Enterprise instance. Seems like all that needs to be added is a param for
--hostname and update the script file.

I'm gonna try to get a PR in for this. 🤞

Examples:

- gh/setup:
          context:
            - GITHUB_CREDS
          hostname: enterprisehostname

Supporting Documentation Links:

GitHub cli docs for login https://cli.github.com/manual/gh_auth_login

[KyleTryon]

PR #19 opened to address this. Nearly ready, but looking for feedback due to current lack of ability to test.