Support for unplugged keyword in .script files.

User progress can now show unplugged level(s) in same stage as other levels.  That view remains compatible with legacy approach which is that stages with a single level have them listed as unplugged.
would it make more sense to have the unplugged property on the level instead of the script_level? If you want a convenience method on script_level you can always def unplugged? self.level.unplugged