Add support of :engine or :class as an alternative to :zen/tags

[niquola]

The tag's system is flexible, but sometimes it is looks too complicated and simple :class or :engine pattern may work:

{ns myapp
 db {:engine pg/db :user "..."}
 db {:zen/tag #{pg/db} :engine pg/db ...}}

:engine or :zen/engine may be indexed as a tag - i.e. (get-tag ztx 'pg/db) will find it