Next On Target Click

Added nextOnTargetClick option, allowing the consumer to specify whether the tour should advance when the target is clicked.

Also added the optional async validateNextOnTargetClick callback, which determines if the tour should continue after the target is clicked. Useful if a target button does some fetching or rerendering that the next step relies on.

Updated customNextFunc to accept a second, optional fromTarget argument. This can be used to produce different behavior depending on how the next function gets called. Could be used to create "optional" steps that are only reached if the user decides to interact with the target.