def add_rule(rule)
      if rule.is_a? Axlsx::ConditionalFormattingRule
        @rules << rule
      elsif rule.is_a? Hash
        @rules << ConditionalFormattingRule.new(rule)
      end
    end
Add a ConditionalFormattingRule. If a hash of options is passed
in create a rule on the fly.
@param [ConditionalFormattingRule|Hash] rule A rule to use, or the options necessary to create one.
@see ConditionalFormattingRule#initialize