Add TeX -> MathML conversion via ritex

This allows math environments to be output as MathML instead of MathJax script tags when using the Html converter. This allows browsers to natively display mathematics without the need for any javascript library (MathJax).

It has been provided conditionally similarly to coderay, with a `enable_ritex` option that defaults to false. It supports both inline and block math.

It relies on the ritex package (`[sudo] gem install ritex]`). I have done my best to keep to the syntax as I could determine it and have not made any changes to the tests or manuals/docs which might be necessary to accept this pull request.

TeX such as `$$f(x) = a{x^3} + b{x^2} + cx + d$$` (a cubic function) would be rendered as:
```html
<math xmlns="http://www.w3.org/1998/Math/MathML" display="block"><mi>f</mi><mo stretchy="false">(</mo><mi>x</mi><mo stretchy="false">)</mo><mo>=</mo><mi>a</mi><mrow><msup><mi>x</mi><mn>3</mn></msup></mrow><mo>+</mo><mi>b</mi><mrow><msup><mi>x</mi><mn>2</mn></msup></mrow><mo>+</mo><mi>c</mi><mi>x</mi><mo>+</mo><mi>d</mi></math>
```
which renders as ![f(x)=ax3+bx2+cx+d](https://raw.githubusercontent.com/tmthrgd/jekyll-plugins/master/mathml.png)