Adam Finn Tulinius

When updating multiple hosts, it would be neat if morph could keep pushing data to the remaining hosts in the background, while waiting for the current host to switch configuration/reboot/&c....

E.g. `morph build examples/simple.nix examples/secrets.nix` (e.g. `morph build examples/*` expanded by the shell). This should of course be support by all subcommands.