Add Grammatical Framework

The Grammatical Framework (GF) is a programming language for multilingual grammar applications. http://www.grammaticalframework.org/

Examples of GF usage on GitHub:
- https://github.com/GrammaticalFramework/GF
- https://github.com/johnjcamilleri/Maltese-GF-Resource-Grammar
- https://github.com/Attempto/ACE-in-GF
- https://github.com/GF-Estonian/GF-Estonian