Add support for multiple media types

repo.read() function should have option for choosing the format of the receiving data. (base64)
For example, if requested target is image file and if you are going to show it on browser then it could be much easier to download content as base64 image so the image url could be easily generated when there is no need to convert it to base64 anymore..

and for receiving / sending commits, pulls, etc.. it would be nice to have an option for choosing diff / patch media type.

https://developer.github.com/v3/media/

https://developer.github.com/changes/2012-12-10-Diff-and-patch-media-types/

Start at the repo.read() entry point and trace how requests for images, commits, and pulls choose their response media types. Read the linked GitHub media-type documentation first, then define how callers select base64, diff, or patch responses and verify each requested format is returned.

索引モデルが issue の本文から書いたものです。