Added credentials for GitHub Resolver

[shubhbapna]

fixes #118

If GITHUB_TOKEN is defined in the current environment then use it to create an authenticated instance of GitHub client and then use it in the resolver.

[tiran]

Good work!

Could you please also update customization.md and mention the env var there? Thanks!

[dhellmann]

Good work!

Could you please also update customization.md and mention the env var there? Thanks!

There's an authentication section in the REAMDE. That might be a better spot for docs.

[shubhbapna]

Good work!
Could you please also update customization.md and mention the env var there? Thanks!

There's an authentication section in the REAMDE. That might be a better spot for docs.

Sorry I don't see an authentication section in the README

[dhellmann]

Good work!
Could you please also update customization.md and mention the env var there? Thanks!

There's an authentication section in the REAMDE. That might be a better spot for docs.

Sorry I don't see an authentication section in the README

D'oh, that PR didn't merge, yet, sorry. #108

[shubhbapna]

I wasn't sure if adding documentation for this would be right in Using private registries so I created another section which specifically states why a github token might be needed and how to pass it in Determining versions via GitHub tags

Let me know if that works @dhellmann @tiran

[dhellmann]

That looks good to me, thank you!

I'll leave this open for @tiran to re-review, since he had asked for changes.