Add a pending state

[devingray]

Hi there. I wanted to add a PR to include a method that checks if the item is pending

OnboardFacade::addStep('is-approved')
      ->pendingIf(function (User $user) {
             return $user->isAwaitingApproval();
         })
       ->completeIf(function (User $user) {
           return $user->isApproved();
       })

This will allow to display a seperate state in the frontend without too much additional logic on the template, it will also allow for filtering by complete / pending states

I can draft up a PR if this seems like a good idea to the maintainers?