Reference/Query trigger status

[jesselawson]

Like switches, we should be able to reference a trigger to determine its state.

For example:

story.registerTrigger({
  label: 'created trigger feature',
  trigger: () => { /* something */ },
  then: () => { /* something else */ }

We should be able to query the status of that trigger, like this:

story.triggerStatus('created trigger feature')
// Returns false

or

story.fired('created trigger feature')
// Returns false