Hello!

I've taken a stab at adding support for allowing
org-columns-summary-types entries to contain an optional COLLECT
function that can be used to conditionally collect a property.

Please see the included patch. I added a NEWS entry, but wasn't sure
whether a manual entry is required.

I added a couple simple tests, which pass, but the test immediately
above the two I added fails when this patch is applied; I haven't been
able to figure out why. Any help appreciated.

Comments / feedback welcome :-)