Support of the .pro file extension for Prolog.

This adds the support of the .pro file extension for Prolog.
This extension is often used as a replacement for .pl (which is also the Perl file extension).
Currently, Prolog files with the .pro extension are detected as IDL files.
This search result displays some misdetected files:
https://github.com/search?l=IDL&q=%3A-&ref=advsearch&type=Code
I've added some heuristics and a sample file.