Adds a new output jax "PlainSource" which exposes a "plain text" source when possible.
More precisely, this jax creates the following output:
- TeX or asciimath if either is used as input
- TeX or asciimath if MathML is used as input and includes annotationXML for either of these. If both are found, TeX is preferred.
This also fixes https://github.com/mathjax/MathJax/issues/788