Support generating text message body for string schemas

[kylef]

This PR adds support for generating message body assets for text types like text/plain and text/html.

Example usage:

responses:
  200:
    content:
      text/plain:
        schema:
          type: string
          example: The request was successful

To produce message body, such as if the response was:

Content-Type: text/plain

The request was successful

[kylef]

I've refactored isJSONMediaType to use mediaTyper so it is implemented in the same way that isTextMediaType is implemented. I think it can be a bit safer to use mediaTyper than custom regex.

[coveralls]

Coverage increased (+0.03%) to 84.842% when pulling c89d21d on kylef/oas3-text-type into b95481f on master.