Extend Latex support

This patch extends the support for Latex in the following way:

* It introduces a helper function to highlight Latex environments that
  are enclosed with `\begin{something} ... \end{something}`. There are a
  lot of these environments in the Latex language that could get special
  highlighting in the future.
* Certain functions like `\ref` or `\begin` always contain a reference
  or a keyword and they are highlighted as such.
* Chapter and section headlines are written in bold to make them stand
  out more, because they structure the document.
* Since `selector` and `string` have the same color in the default
  theme, the token type `function` is used to highlight Latex commands.
* Add support for the `\url` command