adds support for relative setupPath

[JoshMcCullough]

Passing a relative value to as setupPath would cause a an error when pushing to the git repo. I added a check to resolve the setupPath if it is not absolute.

Before, this ...

const gh = new MockGithub(ghConfig, '_github_mock');

... would cause this ...

Pushing to _github_mock/repo/test/remote/origin
fatal: '_github_mock/repo/test/remote/origin' does not appear to be a git repository
fatal: Could not read from remote repository.

Please make sure you have the correct access rights
and the repository exists.

It could have been fixed by doing ...

const gh = new MockGithub(ghConfig, path.resolve(process.cwd(), '_github_mock'));

... but that shouldn't really be necessary from a user's perspective.

[shubhbapna]

Thank you for your contribution!

[JoshMcCullough]

You're welcome. And I see you already published the package. Thank you! 💯