Extend Latex support

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

* Support for various equation environments and for the verbatim
  environment
* 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
* Add alias support