Problem with highlighting groups per tabpages

The current mode is only cached per window. This will break, if one
switches tabpage. We remember the last mode that was used to create the
highlighting group and compare, if this has changed.

This fixes #670

autoload/airline.vim

@@ -179,10 +179,12 @@ function! airline#check_mode(winnr)
   endif
 
   let mode_string = join(l:mode)
-  if get(w:, 'airline_lastmode', '') != mode_string
+  if get(w:, 'airline_lastmode', '') != mode_string ||
+      \ get(s:, 'airline_lastmode', []) != l:mode
     call airline#highlighter#highlight_modified_inactive(context.bufnr)
     call airline#highlighter#highlight(l:mode)
     let w:airline_lastmode = mode_string
+    let s:airline_lastmode = l:mode
   endif
 
   return ''