Ignore callbacks if not specifed on the model

[JDrizzy]

Related comment/discussion: #662 (comment)

With the addition of touch auditing, I thought maybe a configuration could exclude certain callbacks unless the callback action was specified on the model.

Audited.ignored_default_callbacks = [:touch]

class User < ActiveRecord::Base
   audited
end
# audits on action :create, :update, :destroy

class Company < ActiveRecord::Base
  audited on: [:create, :touch]
end
# audits on action :create, :touch