Add prevent_transition as an exclusive next to permit_transition

[svanhesteren]

Sometimes you have a whole list of state transitions and you just want to disallow one or two instead of writing out all possible combinations.
Like this:

  state_machine_enum :state, prefix: "state" do |s|
    s.prevent_transition(:unavailable, :unpublished)
  end

Having the posibility to spec the inverse would be cleaner, just like rails does it in various places with :allow, :except.
I understand this could be a bit involved since it would be nice to check we don't have conflicting rules. In that case an exception could be raised.

[julik]

Good one! Maybe forbid instead of prevent though?