add g:NERDTreeStatusline option

allows the statusline to be customised for the nerd tree window

doc/NERD_tree.txt

@@ -641,6 +641,8 @@ NERD tree. These options should be set in your vimrc.
 |'NERDTreeSortOrder'|           Tell the NERD tree how to sort the nodes in
                                 the tree.
 
+|'NERDTreeStatusline'|          Set a statusline for NERD tree windows.
+
 |'NERDTreeWinPos'|              Tells the script where to put the NERD tree
                                 window.
 
@@ -893,6 +895,20 @@ Other examples: >
 3. Dirs will appear first, then ruby and php. Swap files, bak files and vim
    backup files will appear last with everything else preceding them.
 
+------------------------------------------------------------------------------
+                                                        *'NERDTreeStatusline'*
+Values: Any valid statusline setting.
+Default: %{b:NERDTreeRoot.path.strForOS(0)}
+
+Tells the script what to use as the |'statusline'| setting for NERD tree
+windows.
+
+Note that the statusline is set using |:let-&| not |:set| so escaping spaces
+isn't necessary.
+
+Setting this option to -1 will will deactivate it so that your global
+statusline setting is used instead.
+
 ------------------------------------------------------------------------------
                                                             *'NERDTreeWinPos'*
 Values: "left" or "right"