copy from image to host (`docker cp -image <image>:…`)

It would be nice if we didn't have to spin up a container just to extract a file or directory from an image.  We can currently accomplish this with:

    $ CONTAINER=$(docker run -v ${HOSTPATH}:/tmp ${IMAGE} cp -rp ${PATH} /tmp/)
    $ docker rm "${CONTAINER}"

On `#docker`, `bkc_` suggesed an `-i[mage]` flag for disambiguating container-vs-image for `docker cp`:

    $ docker cp ${CONTAINER}:${PATH} ${HOSTPATH}
    $ docker cp -image ${IMAGE}:${PATH} ${HOSTPATH}

This would involve an API extension (`/images/(name)/copy`, mirroring `/containers/(id)/copy`), and the `-i[mage]` flag would swap the client between API URLs.  I'll take a stab at implementing this if it sounds reasonable.