NerdTree: make the statusline configurable

Use the g:NerdTreeStatusline variable for configuring the
NerdTree statusline in the NerdTree window

closes #1705

autoload/airline/extensions.vim

@@ -22,7 +22,7 @@ endfunction
 let s:script_path = tolower(resolve(expand('<sfile>:p:h')))
 
 let s:filetype_overrides = {
-      \ 'nerdtree': [ 'NERD', '' ],
+      \ 'nerdtree': [ get(g:, 'NERDTreeStatusline', 'NERD'), '' ],
       \ 'gundo': [ 'Gundo', '' ],
       \ 'vimfiler': [ 'vimfiler', '%{vimfiler#get_status_string()}' ],
       \ 'minibufexpl': [ 'MiniBufExplorer', '' ],

doc/airline.txt

@@ -403,6 +403,12 @@ vim-bufferline <https://github.com/bling/vim-bufferline>
 * determine whether bufferline will overwrite customization variables >
   let g:airline#extensions#bufferline#overwrite_variables = 1
 <
+-------------------------------------                   *airline-nerdtree*
+NerdTree <https://github.com/scrooloose/nerdtree.git>
+
+Airline displays the Nerdtree specific statusline (which can be configured using
+the |'NerdTreeStatusline'|variable.
+
 -------------------------------------                   *airline-fugitiveline*
 This extension hides the fugitive://**// part of the buffer names, to only
 show the file name as if it were in the current working tree.