Include parent argument to be able to add include between other children

[cirosantilli]

It is currently impossible, include children must come before all other children.

E.g. if we had:

index.ciro:

= Animal

== Dog

== Cat

== Bat

and wanted to split Cat out at some point to cat.ciro, it would become:

= Animal

== Dog

\Include[cat]

== Bat

but then Cat would be a child of Dog.

So what we want to write is:

= Animal

== Dog

\Include[cat]{parent=animal}

== Bat

much like we can do for headers.

Currently the included Cat can only be a child of Animal if we change ordering to:

= Animal

\Include[cat]

== Dog

== Bat