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