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

lib/nerdtree/path.vim

@@ -61,7 +61,7 @@ function! s:Path.cacheDisplayString() abort
     endif
 
     if self.isReadOnly
-        let self.cachedDisplayString .=  ' [RO]'
+        let self.cachedDisplayString .=  ' ['.g:NERDTreeGlyphReadOnly.']'
     endif
 endfunction
 

lib/nerdtree/ui.vim

@@ -375,7 +375,7 @@ function! s:UI._stripMarkup(line, removeLeadingSpaces)
     let line = substitute (line, g:NERDTreeUI.MarkupReg(),"","")
 
     "strip off any read only flag
-    let line = substitute (line, ' \[RO\]', "","")
+    exec 'let line = substitute (line, " \['.g:NERDTreeGlyphReadOnly.'\]", "","")'
 
     "strip off any bookmark flags
     let line = substitute (line, ' {[^}]*}', "","")