menu_breadcrumb 8.x-1.0-beta1

Code ported from MBAT sandbox; name changes only.
Fixed outstanding bugs (not recognising external links or home route).
Added Taxonomy Attachment feature, effectively extending active trail by taxonomy membership.
Allowed breadcrumb builder to fail over to others when it can't produce a breadcrumb.
Clarified online documentation in both settings and Help page.
Cleared up Drupal 7-isms in code and documentation.

Pending issues for Beta version: please test & look at:
- cache tags used!
- method of determining taxonomy membership
- binary configuration options & their definition

If quirky upgrading from Alpha version, trying clearing cache (to sift out breadcrumb builder priorities) then reinstalling module (should not be necessary).