Allow a workflow to be assigned an initial state different from the first state declared.

My current use case is a model for outgoing payments with the following workflow:

    workflow do
      initial_state :pending

      state :incomplete do
        event :check, transitions_to: :pending
      end

      state :pending do
        event :hold, transitions_to: :held
        event :confirm, transitions_to: :sent
      end

      state :held do
        event :release, transitions_to: :pending
      end

      state :sent
    end

The payout falls into `:incomplete` state if the prerequisites (which are more than validations alone) are not met. However, a new payout should be born with `:pending` state for reasons beyond my control.

I could of course declare the `:incomplete` state last, but since one of the previous extensions of the workflow gem `include Comparable` and thus allow states to be compared by declaration order e.g. with `>` or `<`, the right place of this state is first. 

The `initial_state` is completely optional. Unless present, the first state declared will automatically become the initial state and therefore this commit is fully backwards compatible and shouldn't break any existing code.

Thanks for considering this addition!