convert mode/paste/iminsert parts to function names.

autoload/airline/util.vim

@@ -1,6 +1,24 @@
 " MIT License. Copyright (c) 2013 Bailey Ling.
 " vim: et ts=2 sts=2 sw=2
 
+function! s:get_val(part)
+  let val = g:airline_parts[a:part]
+  if match(val, '%') > -1
+    return val
+  else
+    return '%{function("'.val.'")()}'
+  endif
+endfunction
+
+function! airline#util#define_section(key, parts)
+  if !exists('g:airline_section_{a:key}') && len(a:parts) > 0
+    let g:airline_section_{a:key} = s:get_val(a:parts[0])
+    for i in range(1, len(a:parts) - 1)
+      let g:airline_section_{a:key} .= s:get_val(a:parts[i])
+    endfor
+  endif
+endfunction
+
 if v:version >= 704
   function! airline#util#getwinvar(winnr, key, def)
     return getwinvar(a:winnr, a:key, a:def)