Reliable argument handling

Currently, we allow users to mix optional, variadic, and required arguments. In order of preference:

1. make this impossible (see below)
2. check at start
3. check when calling the command

We currently do *none* of these. Instead, we have some weird logic in CheckArguments where we stop filling in optional arguments once we realize we'll need the rest to fill in required arguments.

---

Preferred solution:

```go
type Command struct {
Arguments Arguments
// ...
}

type Arguments struct {
Required []Argument
Optional []Argument
Variadic Argument
}
```