Suggestion: SDK for Derek

[alexellis]

Would it be useful to provide Derek as an SDK?

The GitHub auth that we've written has already been useful for the GitHub App in OpenFaaS Cloud and could be useful as a separate package.

Due to forking of Derek over one user not wanting to follow DCO process, would it be advantageous for us to release Derek as an SDK so the functionality can be invoked via other processes such as a regular HTTP server or as suggested in another issue - an AWS Lambda function? #13

If Derek was packaged as an SDK a few things would need to change:

• secrets would have to be read ahead of time into memory and passed in a configuration object
• any log.Fatal* lines may need to be removed
• regression testing would also be needed
• updates to docs for consumption of SDK

[alexellis]

I would image this work being done in small increments with each one being released and deployed starting with a configuration object where we load or parse all environmental variables and secrets in one single place then pass that down the stack to any code that needs it.

[alexellis]

Derek add label: Hacktoberfest

[alexellis]

This PR should make as few as possible refactorings and stylistic changes.