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.

lib/linguist/languages.yml

@@ -785,6 +785,12 @@ GAS:
   - .s
   - .S
 
+GDScript:
+  type: programming
+  lexer: Text only
+  extensions:
+  - .gd
+
 GLSL:
   group: C
   type: programming