*: add explicit layered mode

[cgonyeo]

I think a command like acbuild layer should be supported that would add a new layer into the current ACI.

acbuild exec -- wget http://example.com/very-big-file.deb
acbuild exec -- dpkg -i ./very-big-file.deb
acbuild exec -- rm ./very-big-file.deb
acbuild layer

The above would produce a layered image, but the file very-big-file.deb would not exist in any of the layers.

[jonboulle]

Going to wait until we sort out push before dealing with this. #16