Export columns as child elements

Force cells to be exported as child elements of their row element instead of attributes.

Example:

Sheet name: Arrows

Type	Damage	Effect
Normal	1	None
Fire	3	Burn
Ice	2	Freeze
Light	5	Flare

Default Output:

<data>
  <Arrows>
    <Normal Damage="1" Effect="None"/>
    <Fire Damage="3" Effect="Burn"/>
    <Ice Damage="2" Effect="Freeze"/>
    <Light Damage="5" Effect="Flare"/>
  </Arrows>
</data>

Child Elements Output:

<data>
  <Arrows>
    <Normal>
      <Damage>1</Damage>
      <Effect>None</Effect>
    </Normal>
    <Fire>
      <Damage>3</Damage>
      <Effect>Burn</Effect>
    </Fire>
    <Ice>
      <Damage>2</Damage>
      <Effect>Freeze</Effect>
    </Ice>
    <Light>
      <Damage>5</Damage>
      <Effect>Flare</Effect>
    </Light>
  </Arrows>
</data>