Add support for the GDScript language

References:
* https://github.com/okamstudio/godot/wiki/gdscript
* http://www.godotengine.org/

Some projects using it:
* https://github.com/okamstudio/godot
* https://github.com/Qwertie-/Godot-games

My motivation for adding it: To disambiguate these .gd files from GAP .gd files.