Add a TextExtension for processing text/* (e.g. text/plain) responses.

[petejohanson]

Currently, when a response has a Content-Type: text/plain response header, the only way to access the text is to use resource.$response.data directly. It would be nicer if we had a new TextExtension that could handle text content types and populate a special text field with the response body, so instead you can just use: resource.text once the request resolves.

I think by default it should only handle text/plain subtype, but should allow optional parameter for set of subtypes handled, or an empty set if all 'text/*' content types should be handled.