Support default export execution

[orta]

Right now we eval to run the dangerfile, and in most peril setups there's this faff:

const isJest = typeof jest !== "undefined"

// Stores the parameter in a closure that can be invoked in tests.
const storeRFC = (reason: string, closure: () => void | Promise<any>) =>
  // We return a closure here so that the (promise is resolved|closure is invoked)
  // during test time and not when we call rfc().
  () => (closure instanceof Promise ? closure : Promise.resolve(closure()))

// Either schedules the promise for execution via Danger, or invokes closure.
const runRFC = (reason: string, closure: () => void | Promise<any>) => schedule(closure)

const rfc: any = isJest ? storeRFC : runRFC

Which is needed to write tests, and to setup the schedule system which isn't needed in sandbox'd environments like peril-staging but we can verify and wrap it in a scheduler anyway on heroku instances

But what if instead you could provide a default export and that is evaluated or tested? Then you get logical async scope easily, and can test trivially too.

export default async () => {
  if(danger...
}

Might be even better for the DSL because the webhook could be passed in which fixes the awkward const gh = (danger.github as any) as Create too

export default async (status: Status) => {
  if(status.id...
}

[orta]

Support is in Danger

[orta]

And Peril support is in #247