Fixes provide-skin and provide-theme for custom CSS.

Fixes: https://github.com/LightTable/LightTable/issues/983

This may not be the best solution. It requires a boolean flag for skins and themes that require the app://lighttable protocol instead of file://. I thought that was the lesser evil between making the custom styles and the native styles do the extra legwork, but I have a hunch there is a good way to auto-detect native styles that I haven't figured out yet. I'm opening the PR to put the subject up for discussion. If nothing better crops up then this will certainly do the job, it just doesn't feel quite as elegant as it probably could be.

Of particular note is the movement of where adjust-path is called. In the old location (skin-style and stylesheet) by the time it was called (::dir object/*behavior-meta*) was no longer set to the appropriate value, so everything was getting normalized under the lt-home directory.