Fix highlighting problems with Vim

1) Make sure airline_error and airline_warning highlighting are
   different, so that the correct separator will be drawn. This
   fixes #982.

2) allow to deactivate %(%) to workaround a vim bug, that may cause
   leaking of colors from one section to the next and adding additional
   spaces. This needs to be fixed upstream:
   https://groups.google.com/d/msg/vim_dev/sb1jmVirXPU/mPhvDnZ-CwAJ
   Possibly, also related to neovim/neovim#4147
   Use `:let airline#extensions#default#section_use_groupitems = 0`
   to disable grouping of statusline items

doc/airline.txt

@@ -285,6 +285,10 @@ configuration values that you can use.
       \ [ 'x', 'y', 'z', 'error', 'warning' ]
       \ ]
 <
+* configure the layout to not use %(%) grouping items in the statusline.
+  Try setting this to zero, if you notice bleeding color artifacts >
+  let airline#extensions#default#section_use_groupitems = 1
+<
 -------------------------------------                     *airline-quickfix*
 The quickfix extension is a simple built-in extension which determines
 whether the buffer is a quickfix or location list buffer, and adjusts the