Add global filter for wp_list_pages excludes

[markjaquith]

Currently, the only way to reliably exclude pages from wp_list_pages() is to use the exclude query param in each instance of the function. Not user-friendly, as it requires template editing.

There should be a global filter for this. I'll write the patch in a bit.

The filter should probably pass an array of pages already excluded via the exclude= method, and then plugins can just $excludes[] = 34; return $excludes; and we'll implode()

patch for trunk

Patch is up. Will let it sit for a bit, so speak now.

Kool with a K, which is kooler than cool.

(In [4586]) global exclude filter for wp_list_pages(). fixes #3402