add g:airline_exclude_preview option. resolves #72.

autoload/airline.vim

@@ -59,6 +59,9 @@ function! s:is_excluded_window()
       return 1
     endif
   endfor
+  if g:airline_exclude_preview && &previewwindow
+    return 1
+  endif
   return 0
 endfunction
 

doc/airline.txt

@@ -86,6 +86,13 @@ values):
   let g:airline_exclude_filetypes = [] " see source for current list
 <
 
+* defines whether the preview window should be excluded from have its window
+  statusline modified (may help with plugins which use the preview window
+  heavily)
+>
+  let g:airline_exclude_preview = 0
+<
+
 ==============================================================================
 CUSTOMIZATION                                        *airline-customization*
 

plugin/airline.vim

@@ -22,6 +22,7 @@ call s:check_defined('g:airline_paste_symbol', (exists('g:airline_powerline_font
 call s:check_defined('g:airline_theme', 'dark')
 call s:check_defined('g:airline_exclude_filenames', ['DebuggerWatch','DebuggerStack','DebuggerStatus'])
 call s:check_defined('g:airline_exclude_filetypes', [])
+call s:check_defined('g:airline_exclude_preview', 0)
 call s:check_defined('g:airline_window_override_funcrefs', [])
 
 call s:check_defined('g:airline_mode_map', {