Media types for GitHub API previews

[robbiemcmichael]

Adds support for the media types used by API previews. This will allow users of this library to use the existing GenRequest type and its constructors when writing functions that use endpoints from API previews.

[phadej]

I don't feel I'd like to keep that preview list explicitly in the library.

[robbiemcmichael]

I had a go first at refactoring things to allow the media types externally, but it started to get seriously messy. I was hoping you might accept having the list of media types in this library because those strings should at least be stable, but maybe I'll have to give the refactoring another shot instead.

[phadej]

I think we can change MediaType into MediaType ext or make it somehow extensible.
I'll try to make new github release soon, and this is something which would be nice to have; so even more stuff can be done outside the library.

I'll see what I can do, Draft Pull Requests seems to be something which is easy to test with.

[phadej]

Superseded by #370