'Simple' artifacts based on package specifications

It'd be cool to have a sort of file spec for building simple Vorpal artifacts, similar in nature to RPM specs files.

The idea, using the artifact structure in the README as a base for this, would be a file that looks like this:

```toml
# ./Vorpalfile
vorpalfile = 1

name = "example"
systems = ["aarch64-linux", "aarch64-macos"]

[[sources]]
name = "example"
path = "."

[[steps]]
entrypoint = "/bin/bash"
script = 'echo "hello, world!" > $VORPAL_OUTPUT/hello_world.txt'
```

Then, you could just use Vorpal's CLI to read this as a form of SDK input, sort of like this;

```
$ vorpal artifact --from-file=./Vorpalfile
...
```

This would be an interesting thing to play with I think, and would probably help simpler projects adopt Vorpal, before they need to adopt an SDK.