[FR] Add Step Validation

[brennanmann]

@jcmcneal Would you be willing to accept a PR that introduces the concept of step validation?

• validation would be exposed via a callback fn passed in as a prop. The validation function would accept a Boolean value to indicate whether the step is in valid state
• if a value is not provided, the default value would be true for the current step being evaluated
• validation checks may prevent the wizard from advancing to the next step
• ...

If you are interested in adding such functionality, we can hash out the details.

[iamwilson]

This feature will sure come in handy.