Conditionals in DAG spec

[EpicWink]

Support conditionals in DAG-type workflow-spec.

Sometimes activity tasks in workflows only need to be run if a certain condition is met. There is no way to have optional branches in the current workflow spec definition.

Proposed syntax example:

tasks:
  - id: foo
    type:
      name: spam-foo
      version: "0.3"
    if:
      from: workflow
      key: $.run-workflow
  - id: bar
    type:
      name: spam-foo
      version: "0.4"
    dependencies:
      - foo
    if:
      from: dependency
      key: $

Nothing more complex should be required as a prior task could be used to use the entire current workflow state to determine if a branch should be run.