Allow outputting an image layout instead of an image

[kamalmarhubi]

I'm working on aproject where I'm going to use docker2aci only to immediately extract the ACI into an image layout. It would by handy to enable docker2aci to output an image layout directly and save a bunch of useless disk IO.

I'm not sure if this fits docerk2aci's mission includes this, but it does seem like a good place to put that functionality.