Adds accessor method prefixes

- Adds prefix methods for model accessors
- Implements array setters
- Implements << setters

        class MyModel
          extend Bitmasker::Model

        has_bitmask_attributes :roles, 'has_role_' do |config|
          config.attribute :admin, 1 << 0
          config.attribute :manager, 1 << 1
         end
        end

        my_model.roles << :admin
        my_model.roles = [:admin, :manager]

        my_model.has_role_admin? # true
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