Ability to trigger tasks from other tasks

[jzone3]

Not yet finished

TODO:

• [x] Add triggered_by argument into @task decorator
• [ ] Validate that all task keys are valid in build step
• [ ] Check for cycles in build step!
• [ ] Add tests
• [ ] Build the DAG

NOTE: I switch from the planned triggered_by=[a,b] to triggered_by=["a", "b"] because ast evaluates the literal code so the object is not actually passed... duhhh