add a path filter API

Add an API to allow custom "path filter callbacks" to be added.

Previously we allowed one path filtering function to exist called
`NERDTreeCustomIgnoreFilter`. This has been removed and replaced with an
API to allow any number of such functions to exist -  via the new
`NERDTreeAddPathFilter()`

plugin/NERD_tree.vim

@@ -196,6 +196,11 @@ function! NERDTreeCWD()
     call NERDTreeFocus()
     call nerdtree#ui_glue#chRootCwd()
 endfunction
+
+function! NERDTreeAddPathFilter(callback)
+    call g:NERDTree.AddPathFilter(a:callback)
+endfunction
+
 " SECTION: Post Source Actions {{{1
 call nerdtree#postSourceActions()