Fix variable name conflict with Compass v0.13.x

When using box-shadow() from Compass v0.13.x, the grid-span() stops working because of a naming conflict the set-arglist-default function: it also uses the $output variable name (see: https://github.com/chriseppstein/compass/blob/master/frameworks/compass/stylesheets/compass/css3/_shared.scss#L261).

I suggest to change the $output variable to $output-style