Add options for repository media types

[vmolsa]

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

example:

repo.read('master', 'README.md', { contentType: 'base64' }, function(error, data) {
  console.log(data.content);
});

[aendra-rininsland]

Terrific! This totally closes #218. Could you please add a unit test or two? Will gladly merge after. Thanks! :+1:

[aendra-rininsland]

Bumping to 0.10.8, we really need to release 0.10.7.

[aendra-rininsland]

Closed in error; reopening.