Fallback support for popup steps when element is not found

[craig0990]

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.