Add Math Goodie to render TeX with MathJax

Renders math using MathJax.

Eg. A search for `$\sum_{i=0}^{N} x_i$` or `\(\sum_{i=0}^{N} x_i\)` will load MathJax from the official CDN (using SSL) and render the equation.

See here for original feature request: https://duckduckhack.uservoice.com/forums/5168-ideas-for-duckduckgo-instant-answer-plugins/suggestions/2679072-latex-rendering-give-a-rendered-image