Chained commands with children will now print options

Quick fix to allow the user to hit enter on the parent of a chained command and see the child options. For example, if: 

    showcmd = console.addChild(Command('show'))
    showcmd.addChild(Command('disk')).addChild(DiskIO('io'))
    showcmd.addChild(Command('network')).addChild(DiskIO('io'))

then:

    > show [RET]
    Options: disk, network

Fixes #12