Deep clone fix to keep stylus nodes intact

This pull request is a fix for #101.

Since paths is the only value of opts that we mutate whose changes would affect the outside, I've added special handling for it so we don't lose the functionality introduced in 15fb57a81bfcbf927a84c14e364a2889128997d8.

@TeffenEllis it would be great if you could take a look and let me know if it works for your use case.