"pause-before' policy

We need a policy to be able to pause workflow execution before each task. So if this policy applies workflow is going to be managed manually.
Basically, we just need to implement policy 'pause-before' with the ability to evaluate expressions like as follows (so that we could control it either from DSL and API if needed):

  wf1:
    type: reverse
    input:
      - param1
      - param2
      - pause_before

    task-defaults:
      policies:
        pause-before: $.pause_before