Fallback support for popup steps when element is not found#56

Current proposal is a `fallback` option, which accepts either `modal` or `skip`, and defaults to `modal`. This option will only apply to `popup` steps. If the element defined by a `popup` step does not exist and `fallback` is `modal`, then a `modal` style step will be displayed instead. If the `fallback` is `skip`, the next step will be displayed instead. Care must be taken around `onEnter` and `onLeave` callbacks.