Implement function call highlighting

Closes #63

doc/python-syntax.txt

@@ -92,6 +92,9 @@ following command to your `~/.config/nvim/init.vim` or `~/.vimrc`: >
 `g:python_highlight_doctests` (default `0`)
     Highlight doc-tests
 
+`g:python_highlight_func_calls` (default `0`)
+    Highlight functions calls
+
 `g:python_highlight_class_vars` (default `0`)
     Highlight class variables `self` and `cls`