Support of the .ecl file extension for Prolog.

This adds the support of the .ecl file extension for Prolog.
.ecl is used for ECLiPSe Prolog files (as opposed to pure-Prolog files).
This search result displays misdetected files:
https://github.com/search?l=ECL&q=%3A-&ref=advsearch&type=Code
One of the best example being:
https://github.com/hakank/hakank/blob/d0de1db0f7b1f87847b2b2702dbcedd86cb7a5ff/eclipse_clp/discrete_tomography.ecl

The ECL language also uses this file extension thus I added some heuristics and a sample file to go with it.