Allow user-defined RO indicator.

(inspired by overwriting g:NERDTreeDirArrowCollapsible/Expandable)

Replace instances of the RO string with a variable that the user can
override. Useful for custom unicode glyphs, i.e. Font Awesome.

* Initialize variable g:NERDTreeGlyphReadOnly = "RO".
   -> plugin/NERD_tree.vim
* Replace instances of 'RO' with g:NERDTreeGlyphReadOnly

plugin/NERD_tree.vim

@@ -84,6 +84,8 @@ else
     endif
 endif
 
+call s:initVariable("g:NERDTreeGlyphReadOnly", "RO")
+
 if !exists('g:NERDTreeStatusline')
 
     "the exists() crap here is a hack to stop vim spazzing out when