add box-shadow support.

Author: jaywcjlove

lib/vendor.styl

@@ -70,6 +70,9 @@ box-pack()
 border-radius()
     vendor('border-radius', arguments, only: webkit official)
 
+box-shadow()
+  vendor('box-shadow', arguments, only: webkit official)
+  
 border-image()
     vendor('border-image', arguments, only: webkit moz o official)