Add docker squash command

This adds a new cli command like:
docker squash baseimage leafimage

This command creates a new image that is a child of baseimage
and has the same content as leafimage. In other words, it combines
all the layers between baseimage and leafimage into a single
image.

There are several reasons why this is useful, for instance it is common
for intermediate layers to add extra files during execution which are
removed at the end (for instance build dependencies, or e.g. yum/apt-get
metadata). Removing these makes for a smaller final image.

Docker-DCO-1.1-Signed-off-by: Alexander Larsson <alexl@redhat.com> (github: alexlarsson)