Feature request: color

It would be nice to be able to convert color attributes.

For example, from MathML
```{mathml}

```

to/from LaTeX (assuming the color or xcolor package is loaded...)

```{latex}
$$ {\color{blue}m}x+{\color{red}c} $$
```
which renders as

$$
{\color{blue}m}x+{\color{red}c}
$$

Alternatively, (although MathJAX doesn't support this)

```{latex}
$$ \textcolor{blue}{m}x+\textcolor{red}{c} $$
```