important math bugs

[cben]

• [ ] $x_y$ should not make the rest of the line italic [https://github.com/cben/CodeMirror-MathJax/issues/4]
• [ ] Edit formula on click [https://github.com/cben/CodeMirror-MathJax/issues/9]
• [ ] unrendering/rerendering is flaky [https://github.com/cben/CodeMirror-MathJax/issues/23]

Would be great to do https://github.com/cben/CodeMirror-MathJax/issues/13 and everything following from it, especially:

• [ ] multi-line equations [https://github.com/cben/CodeMirror-MathJax/issues/3]

but that's less urgent.

[cben]

Also consider performance:

• [ ] KaTeX [https://github.com/cben/CodeMirror-MathJax/issues/39]

and/or

• [x] MathJax 2.5 [https://github.com/cben/CodeMirror-MathJax/issues/33]

[cben]

New MathJax finally addressed by #169