Bug #2180, Optionally extract the quiz ID from the URL - SlickQuiz

* (Commit attempt 2, https://github.com/jewlofthelotus/SlickQuiz-WordPress fork)
* A backwards compatible change, that guards against mis-spellings
* Shortcode pattern(s):  [slickquiz id=url], [slickquiz id=(url|uri|-1)]
* https://plugins.trac.wordpress.org/ticket/2180
documentation, version bump, standardized variable checks