Add total scrobbles to Tracks chart

Hi chocolateboy,

thanks for the sort.fm userscript! I've been using a modified version
of it to inform my listening habits towards new artists. My version
adds an extra column to the artist's Tracks chart, showing the total
playcount for each track. I sometimes find this to be a better measure
of a track's popularity.

Maybe this can be of use to you too? See the commit message for
technical details.