Add support for the Rouge syntax highlighter

By setting the `highlighter` setting to `rouge` you can now easily
highlight your code with it instead of relying on Pygments. However,
Jekyll doesn't depend on Rouge explicitly, you will need to install it
or add it to your Gemfile.

The documentation has been updated accordingly.
Set highlighter to pygments when upgrading

In case you are upgrading from 1.4.2 to 2.0 and the pygments option is
set to true, then the highlighter option will be set to pygments
automatically.
Rely on the Redcarpet plugin instead of hard-coding

To avoid code duplication and have to keep tracking of the API change of
Rouge, let's rely on the Redcarpet plugin and customize the output on
our needs.
Use Rouge::Formatter#format instead of #render
Make the pre regex more laxist to match classes

Since Rouge is automatically adding a class attribute to the generated
pre tag, the regex passed to the add_code_tags method should match it
when highlighting through the `highlight` Liquid tag