important math bugs

[cben]

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

Would be great to do cben/CodeMirror-MathJax#13 and everything following from it, especially:

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

but that's less urgent.

[cben]

Also consider performance:

• KaTeX [https://github.com/KaTeX support (with or without MathJax fallback) CodeMirror-MathJax#39]

and/or

• MathJax 2.5 [https://github.com/[Math Processing Error]s with MathJax 2.5 => can't typeset detached CodeMirror-MathJax#33]

[cben]

New MathJax finally addressed by #169