The urls setup of old gave us various arguments at call time for
custom_index(), and some where useful. Hopefully, listing() can support
the same type of system, over time, once adapted. Here's an attempt to
get it started, first by supporting paginate_by.
I don't believe custom_index() should be used anymore given that we have
this new listing setup. At least, that's what I surmise based on
current error messages. I may bring this back if removing it breaks
something else.