Add option for skin.directory specification in skin registration

I like using git submodules for my projects and having loveframes as a git submodule prevents me from placing my custom skins within the loveframes/skins/ subdirectory. This small adjustment allows the user to override the default skin directory during skin registration.

Putting something like this in my skin definition works quite beautifully with this small adjustment.
```lua
skin.directory = select(1, ...):gsub('%.','/'):gsub('/[^/]+$', '/')
```