Added YAML Support to Examples

[adam8797]

I've added support for sending back YAML examples if the application supports it.

I've also gone and encapsulated the logic around the individual formats, instead of checking the type in the rendering methods

One of the things that bugs me about this is that YAML is whitespace sensitive, and in order to actually get it to render properly in the document we have to render it as an OpenApiString and let the framework add newlines for us.

Existing tests pass, but I haven't added new tests or examples yet.

[mattfrear]

Looks good so far, nice one. I can't say I've ever seen an API which returned YAML so it seems like an edge case to need examples in YAML.

Could you add some tests?

[adam8797]

Sure thing. Just wanted to get a stamp of approval in the implementation code before I went and did the rest of that work.