That's a good idea!
I don't know how to do it right now, but I put it in my to-do list.
There is an implementation of it in 0.2.1 build.
For now, it's a test, you have to right-click on pipette-tool to launch it, wait a bit (sometimes) and after, when you pick up color on a transparent part of the canvas, you get the color behind...
