Add a "slim" variant

DE Veteran Unix Administrator Needs no documentation. VUAs know their stuff or they know where to find info outside of the container.

The official Debian images also provide a "slim" variant and based on the jessie sizes, it would be possible to slash of some 20+MB (about 40%) of the image. The required tweaks are available from the debuerreotype code's debuerreotype-slimify script. In addition, it would be nice to treat Recommends: as not important (and purge them when no longer needed).

A "slim" variant can be prepared by further modifying the content of the rootfs (after that has been tarred up for the regular image!). This implies that the scripting gets appended to bootstrap.sh. Scriptlets should be put in scripts-slim/, analogously to the ones in scripts/.