autolabor: Allow improved behavior for "artisan" labors

Autolabor's default behavior isn't great for labors where you really want a few skilled craftsdwarves. This is a simple way to adjust all such labors. The most common use cases would be:

    autolabor-artisans disable (for manual management of these labors)
    autolabor-artisans 0 2 (to set a low limit)