Add support for Github Enterprise

I’m trying to use https://github.com/siddharthkp/bundlesize which depends on this library in Github Enterprise.

Looking at the code

    url: `https://api.github.com/repos/${build.repo}/statuses/${build.sha}`,

Should be(in my case):

    url: `https://github.ibm.com/api/v3/repos/${build.repo}/statuses/${build.sha}`,

You can probably accept another parameter in the constructor with the GitHub API URL. I have also seen people using environment variables like: GITHUB_API.

Thanks for the great work you are doing.

[siddharthkp]

@JimmyMultani there’s a PR for this: https://github.com/siddharthkp/github-build/pull/5

I don’t have any experience with GitHub enterprise to be able to help, If you’d like you can take it forward

[JimmyMultani]

@siddharthkp I’d like to take this up, if you’re good with that.