Added "--add" command line option to add tar files as docker layers.

[Vbitz]

The --add option allows tar files to be added to docker containers without extracting inside the container or using intermediate layers.

For example if you have a rootfs in .tar.gz format named hello.tar.gz you can use the following command to add it.

neurodocker generate docker -b ubuntu -p apt --add ./hello.tar.gz /

This option is not currently supported with singularity.