\newcommand's influences should NOT be persistent

[Menci]

If we use \newcommand in a TeX math, the influences will be persistent in current process.

The expected result is -- all \newcommands should only affect the current requested page.

The harm is that if I run a randering server with this library, a malicious person can submit a request that replaces builtin commands with their owns with \renewcommand, so later requests won't return correct result.

This upstream PR may help: mathjax/MathJax-node#348.