Add support for GitHub Enterprise

To use it, set the GH_CLI_URL environment variable -- e.g.:

    export GH_CLI_URL='https://github.mycompany.com'

This allows me to use `gh` with our GitHub Enterprise server -- e.g.:

    $ gh -r devmonkeys/smstack issue.ls | head -n 2
    #257  Add gui - @sontek
    #254  Make it fast - @georges

It's a pain to switch back and forth between GitHub Enterprise and public GitHub, because there is only one `~/.githubconfig` file with one `[github]` section to store the token. Perhaps in another PR, I can make it grow the ability to store tokens for multiple servers.