Added `cls` to conventions

http://stackoverflow.com/questions/18489180/convention-for-my-class-in-class-method

dotfiles/vim/syntax/python.vim

@@ -149,6 +149,7 @@ syn keyword pythonOperator      and in is not or
 
 " added by aloism
 syn keyword pythonConvention    self
+syn keyword pythonConvention    cls
 
 if s:Python2Syntax()
   if !s:Enabled("g:python_print_as_function")