commit command to add to overlay

[progrium]

You can only rebuild an environment from scratch right now. However, for larger environments this build can stall and never finish (#279), but I have been able to manually/interactively install packages without hitting the stall. Having something like Docker's commit was something I've had in mind for a while, but this would provide a workaround for #279.

We dont track individual layers like Docker (yet), so commit would just add files in the scratch overlay to the project overlay/tarball. There would be no undo (maybe removing the files and committing?) and once you use commit, we'd have to disable checking and running envbuild on boot because that would clobber the commits.