Add repl feedback to clj-frontend using EventBus

The clj-frontend ns, see #123, let's you create and run workflows in clojure, but when running a workflow in the repl, there is currently no feedback on the status of the running workflow.  The repl just blocks until the workflow is finished.

This commit adds repl output statements for each step and when the workflow starts and finishes.  The repl feedback can be customized using the :repl-feedback option to clj-frontend/run-workflow with settings :quiet, :default or :verbose.

Internally, the feedback is created by subscribing to events from the embedded EventBus, see #90 and #103.  Thanks to @myronahn for implementing the EventBus and showing me how to use it.

--Morri

cc @donall-factual