Toggle 'STATE_' prefix on autogenerated constants

[GMorris-professional]

Opening a PR to see what the community reaction is to allowing users to turn off the prefix on autogenerated constants.

With the following example state machine:

class SimpleExample < ActiveRecord::Base
  include AASM

  aasm prefix_constants: false do
    state :sleeping, initial: true
    state :running
    state :cleaning
  end
end

Would produce the following constants:
SimpleExample::SLEEPING
SimpleExample::RUNNING
SimpleExample::CLEANING

[codeclimate]

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