Support preview of the current Artboard

This change will save users from having to deal with specially named
slices, making it easier to preview the Artboard they're currently
working on. The 'Preview' slice is still looked for, so existing users
or users who do not use Artboards should be unaffected. Users wishing to
use the new behavior need only delete their 'Preview' export layer.

This change was originally suggested by @davidklaw and refined by
@jelias.