purl module for Drupal 8

This is an effort to replicate the functionality of the purl module for Drupal 7 for Drupal 8.

This is not a direct port. This is written from scratch with Drupal 8's brand new architecture in mind.

Important Notes

1. There is a bug right now that if a path prefix modifier of "node", or "admin" exists, the real /node/*, /admin/* routes will be unreachable.

   Its either we validate against these "known" path prefixes, or change the general approach to something that wouldn't have this limitation (for example, using route providers instead.)

1. This module requires you to patch Drupal Core with: https://gist.githubusercontent.com/bezhermoso/2dd6bade0a57545cbf1a/raw/151534eadc13466ec8cb16a72901a73ca4b63855/url-generator-host-option.patch

   Patch no longer required!