Allow getting README contents in different formats

[BR0kEN-]

For now, with API v3, we can get the contents of README.md in raw format or as rendered html. To do so request should contain Accept header with one of values described in documentation: https://developer.github.com/v3/repos/contents/#custom-media-types.

My proposition - is to add the third argument ($format = 'raw') to the readme() method of \Github\Api\Repo class and empower the request with necessary header.

[Nyholm]

This can be closed.

[BR0kEN-]

PR is merged, - so yep, can be. Thanks.