Allow specification of png fallback path

As [discussed here](https://github.com/filamentgroup/grunticon/issues/58) some users need to have PNG fallbacks served at a different location than their path, e.g. the Rails asset pipeline.

It would be nice to allow to set URLs with this, such as `https://example.s3.amazonaws.com/`, but the split and join [here](https://github.com/filamentgroup/directory-encoder/blob/master/lib/png-uri-encoder.js#L26-L27) will break the protocol and leave you with `https:/example...`