Go-github: Provide capability to specify the `Accept` request header when retrieving issues.

[xcoulon]

There is currently no way to specify a custom mime-type such as html+json when retrieving GitHub issues. However, this is needed when the client wants to retrieve the issue body rendered in HTML rather than in its input/raw format.

This issue has been discussed in https://github.com/google/go-github/issues/6#issuecomment-278333227

[sahildua2305]

@gmlewis what's the status on this?

[dmitshur]

We could probably address this in a similar way to #767 and #481.

[gmlewis]

See also #727.