RFC: Output plain text

[mrtom]

Overview

I'd like to be able to request the output of the Ink parser in a plain text format as well as an HTML format. This PR adds that functionality.

I have only updated a single test so far as a demonstration that it works. I expect this idea will be shot down for performance reasons, so I want to put this out as a litmus test first. I can update all the tests if people are happy with the concept :)