Add grammar for CartoCSS

.gitmodules

@@ -531,3 +531,6 @@
 [submodule "vendor/grammars/sass-textmate-bundle"]
 	path = vendor/grammars/sass-textmate-bundle
 	url = https://github.com/nathos/sass-textmate-bundle
+[submodule "vendor/grammars/carto-atom"]
+	path = vendor/grammars/carto-atom
+	url = https://github.com/yohanboniface/carto-atom

grammars.yml

@@ -136,6 +136,8 @@ vendor/grammars/c.tmbundle:
 - source.c.platform
 vendor/grammars/capnproto.tmbundle:
 - source.capnp
+vendor/grammars/carto-atom:
+- source.css.mss
 vendor/grammars/ceylon-sublimetext:
 - module.ceylon
 - source.ceylon

lib/linguist/languages.yml

@@ -450,12 +450,13 @@ Cap'n Proto:
   ace_mode: text
 
 CartoCSS:
+  type: programming
   aliases:
   - Carto
   extensions:
   - .mss
   ace_mode: text
-  tm_scope: none
+  tm_scope: source.css.mss
 
 Ceylon:
   type: programming