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()`

lib/nerdtree/path.vim

@@ -392,6 +392,12 @@ function! s:Path.ignore()
                 return 1
             endif
         endfor
+
+        for callback in g:NERDTree.PathFilters()
+            if {callback}({'path': self, 'nerdtree': b:NERDTree})
+                return 1
+            endif
+        endfor
     endif
 
     "dont show hidden files unless instructed to
@@ -403,10 +409,6 @@ function! s:Path.ignore()
         return 1
     endif
 
-    if exists("*NERDTreeCustomIgnoreFilter") && NERDTreeCustomIgnoreFilter(self)
-        return 1
-    endif
-
     return 0
 endfunction