Opens an overlay and returns a promise for its acceptance.
It's useful to think of overlays as promises
which may either be fulfilled (accepted) or rejected (dismissed).
Example
Instead of using up.layer.open() and passing callbacks, you may use up.layer.ask().
up.layer.ask() returns a promise for the acceptance value, which you can await:
let user = await up.layer.ask({ url: '/users/new' })
console.log("New user is " + user)
