M140 S0 (Bed cooldown) integration into Custom G-code End Commands

The change that I made in this pull request is to set "turn off the bed once the print is done" (M140 S0) into the default Slic3r Custom G-code End Commands.

When printing, I find it helpful to shut off the bed once the print is done, so that the parts can be removed. I added this command into my setup of Slic3r and find it useful, which is why I think it should be a default command.

Comments and thoughts appreciated.

blue-ice