add NERDTreeCustomIgnoreFilter hook

this gives users a chance to filter out nodes using whatever logic they
please

doc/NERD_tree.txt

@@ -1128,6 +1128,7 @@ The latest dev versions are on github
 
 Next
     - add 'scope' argument to the key map API
+    - add NERDTreeCustomIgnoreFilter hook - needs doc
 
 4.2.0
     - Add NERDTreeDirArrows option to make the UI use pretty arrow chars

plugin/NERD_tree.vim

@@ -2306,6 +2306,10 @@ function! s:Path.ignore()
         return 1
     endif
 
+    if exists("*NERDTreeCustomIgnoreFilter") && NERDTreeCustomIgnoreFilter(self)
+        return 1
+    endif
+
     return 0
 endfunction