FR: `cmd!` should support interpolating lists of arguments, e.g. `Vec<String>`

It'd be nice if this worked:

```rust
let files =  vec!["one", "two", "three"];
cmd!(sh, "cat {files}");
```

The best workaround seems to be to use Rust's stdlib directly, where commands take lists of arguments, but then I have to do my own error reporting.

Or is there a better workaround already?