text: Title: Media handler for `text/plain`
  Body: As proposed by @maxking on Gitter:
  > I have been trying to add support for msgpack in Mailman's API ([mailman/mailman!977](https://gitlab.com/mailman/mailman/-/merge_requests/977)) and in the process of doing that, I am trying to move off of doing the serialization in our code and setting the request.body/text to setting the `response.media` (in the actual resource) and `response.content_type` (before hand, in a middleware using `request.accept`) to let Falcon do the right thing. One thing I started noticing is that we were still returning some plain text and there is no default handler for `text/plain` response (presumably because it can be done with `response.text` or something). I was wondering if y'all would be open to adding a handler for text/plain, which basically works like identity?