Add support for audit association extensions to has_associated_audits

[coalest]

This PR adds support for association extensions to has_associated_audits.

Example usage:

class User < ActiveRecord
  audited
  has_associated_audits do
    def updates
      where(action: :update)
    end
  end
end

user.audits.updates # retrieve only update audits

If there is any interest in this, I can add some documentation and tests to this PR.