Add possibility of children extrarows inside parent extrarow.

[fabiomartins87]

It would be great if the extension could let us create children extrarows inside a parent row. Like this example:

*---------------------------------------------------
Parent Row
*---------------------------------------------------
||||||||| Children Row
||||||||| --------------------------------------------
||||||||| Data | Data | Data | Data
||||||||| --------------------------------------------
||||||||| Children Row
||||||||| --------------------------------------------
||||||||| Data | Data | Data | Data
*---------------------------------------------------
Parent Row
*---------------------------------------------------
||||||||| Children Row
||||||||| --------------------------------------------
||||||||| Data | Data | Data | Data
||||||||| --------------------------------------------
||||||||| Children Row
||||||||| --------------------------------------------
||||||||| Data | Data | Data | Data