Can has traits and null handling pls?

This release brings a degree of internal refactoring. The headline feature is that all configuration constants have been moved to their appropriate Interface, which should hopefully make things more predictable when using this package. If you have not been using the configuration constants and passing strings instead, then this update will not affect you at all.

The other main improvement is that the glide_url helper function will now handle null input gracefully, returning the empty string instead of causing PHP to throw an Exception.