Support token in environment variable

[dbnicholson]

Setting up the token file can be cumbersome in some situations, and passing the token on the command line is insecure. It would be nice to get the token from an environment variable. It would be pretty easy to include. I suggest using GITHUB_TOKEN as that's what github's own tool uses - https://github.com/github/hub/blob/master/share/man/man1/hub.1.ronn#github-oauth-authentication.