Add support for arguments in --help-completion

This extends the current option-only support with support for
arguments too. They are serialised without a without the prefix (so
just with the type) and the min/max ranges (max="none" for unlimited).

A few new option types have been added as well.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>