Add OpenGL shading language (GLSL)

This is the language used for writing OpenGL shaders, which are becoming much more mainstream lately.

lib/linguist/languages.yml

@@ -476,6 +476,15 @@ GAS:
   extensions:
   - .S
 
+GLSL:
+  group: C
+  type: programming
+  primary_extension: .glsl
+  extensions:
+  - .frag
+  - .glslv
+  - .shader
+
 Genshi:
   primary_extension: .kid