Add the ability to handle commands having subcommands, like `qvm-block`
Split the ArgumentCheckVisitor in an OptionsCheckVisitor &
SubCommandCheckVisitor. The OptionsCheckVisitor checks options given
in a section named 'Options' (case insensitive), while the
SubCommandCheckVisitor dispatches on a section named 'Commands' (case
insensitive).
This also fixes finding the undocumented command arguments. The previous
solution with depart_document did not work. NodeVisitor does not dispatch to
depart_document() even if it's mentioned in the documentation.
