PlainText Bug Fix

Add tex2jax_ignore to plainText dislay mode to keep it from being rendered from floating Mathjax.

This addresses https://github.com/openwebwork/webwork2/issues/481

I checked around and it looks like general_math_ev3 does all of the formula processing.  If there is anything else I am missing let me know.  
You probably should do HTML special character escaping, as is done in some of the other modes that insert the mathematics into HTML (like jsMath mode).  That should probably be added to the AsciiMath and LaTeXMathML modes as well, since the math is inserted into the text of the document.

You can also take advantage of the `$in_delim` here (and in the default section below) rather than having to add the delimiters again by hand.