Adding more summary options to values binning

[casblaauw]

Hi, I've been a happy user of pybigtools and specifically BBIRead.values() for a while. Elsewhere in our package we use average_over_bed, and this has a number of useful summary statistics that aren't available in values().
The primary one I'd be interested in is simply sum. I've been assuming values' mean works like mean0 by default if you set missing to 0., but if not having a separate mean0 next to mean would also be great. Is it possible to easily add these binning options values as well?