added cssclass latex command
renamed \cssclass to \class to match MathJax's

http://docs.mathjax.org/en/v2.2-latest/tex.html#html
\class generates HTML templ inst of jQadd mutating DOM

a la 481ae69f6b96ac5df6b17b31b9bbbcb664879144

better for performance reasons:
https://github.com/mathquill/mathquill/pull/191#issuecomment-18180725
cleanup parser variables #cosmetic
whitelist CSS class names allowed by \class

in the eloquent words of faf2eedf0021aa610c68a455944d77ca7c3010e3,
"no injections plz"

kthxbai

owaitbtw the whitelist (to avoid injections) of allowed characters in
CSS class names are from [the CSS grammar][1] (because [HTML allows any
non-space characters in class attributes?!][2]), which besides the
obvious of word characters and dashes, include escape sequences (so
allow backslash) and a certain set of non-ASCII characters.

[1]: http://www.w3.org/TR/CSS21/grammar.html
[2]: http://www.whatwg.org/specs/web-apps/current-work/multipage/elements.html#classes

discussion: https://github.com/mathquill/mathquill/pull/191#discussion_r4326151
Merge pull request #191 from krawaller/dev

(kind of a lie--not straight from krawaller/dev, did some commit
gardening)

add \class as defined by MathJax