redpen: support end_lnum and end_col if possible

test/handler/test_redpen_handler.vader

@@ -10,6 +10,8 @@ Execute(redpen handler should handle errors output):
   \   {
   \     'lnum': 1,
   \     'col': 9,
+  \     'end_lnum': 1,
+  \     'end_col': 15,
   \     'text': 'Found possibly misspelled word "plugin". (Spelling)',
   \     'type': 'W',
   \   },