Support non-json content types?

Hello and thanks for a great project!

I wondered whether there's any interest in support for non-json content types?

Two examples I have right now are that I'd like to annotate some existing methods to document:

- An API that returns no content. The only options I seem to have are `None` which comes back as `null` and an empty string, which comes back as `""`.
- A simple string value. I've refactored a method that used to be equivalent to `return web.Response('foo')` to be `return APIResponse('foo')` but over the wire, I'm changing the API behavior. It used to be returning `foo` but now it returns `"foo"`.

This assumption that all API respond in json requirement seems to be coming from [here](https://github.com/aio-libs/aiohttp-apischema/blob/master/aiohttp_apischema/response.py#L31-L37)?

Thank you!