Pass (row, col) to valueFormatter

It's a little crazy that this information isn't already passed through to these callbacks. See the SO question linked to in the issue below. I'd like to include this in a 1.1.1 minor update.

Fixes #520 
I was worried about situations where only some series have a point at this x-value. In that case `sel_points.length != labels.length`. Of at least I think that's the case.