Add a planning provider for GitHub

@alexcouper this is just so that you know what I am doing and where I am heading, and add your comments before I stray too far.

## What I have thought and done

I have thought that a good use case for deploystream is to help us manage the development of itself. So the first implementation of a planning service is going to be Github issues. Then I can start getting a list of features to render in the frontend etc.

While working on it I realised that we hadn't added the possibility to retrieve a list of features from anywhere, so I put it in the Planning service. 

## What I think we need to do aditionally

When doing so I realised that the features themselves need to store a reference to the planning plugin where they came from, and realised that if we are always going to retrieve features from some planning tool, what we had labelled as planning info maybe are just regular fields on `Feature`.