support specifying multiple deployment files

[adamtulinius]

trafficstars

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.

[johanot]

One problem here is bash completion. Consider morph deploy examples/default.nix ... switch. It's impossible to completion suggest multiple file args, while also being able to suggest keyword args like "switch" | "dry-activate" etc. with kingpin. We might have to make a compromise here.