introduce new warning section, and refactor whitespace into an extension which uses it

autoload/airline/extensions/whitespace.vim

@@ -0,0 +1,11 @@
+" MIT license. Copyright (c) 2013 Bailey Ling.
+" vim: ts=2 sts=2 sw=2 fdm=indent
+
+function! airline#extensions#whitespace#check()
+  let w:airline_section_warning = '%{get(w:, "airline_active", 0) && search(" $", "nw")'
+        \ .'? " ".g:airline_whitespace_symbol." " : ""}'
+endfunction
+
+function! airline#extensions#whitespace#init(ext)
+  call a:ext.add_statusline_funcref(function('airline#extensions#whitespace#check'))
+endfunction