Merge: nitiwiki: external highlighter

Merge: nitiwiki: external highlighter

This is a cleaned up version of what is currently doing the code highlighting and the table in the deployed http://nitlanguage.org

The basic idea here is to delegate the job to some other program. The program has to be configured in the config.ini under the key `wiki.highlighter`

This approach, while not pure nit, makes it quite easy to use whatever highlighter program the user prefer with whatever options (instead of hard-coding a specific external program in the nit source code).

An advanced way is to delegate to a shell script that checks arguments then dispatches to a real program.

ping maintainer: @Morriar

Pull-Request: #1701
Reviewed-by: Lucas Bajolet <r4pass@hotmail.com>