Struct subcommands

- Required fields must be in order and have no key.
- Optionals must have a key.
- Vec (or unpredictable sized) types can not have a value after it, including optionals.

```rust
enum Actions {
  Spawn(SpawnAction),
}

struct SpawnAction {
  Actor: ActorType,
  Position: Vec2,
  Health:  Option<u32>,
}
``` 

```
/spawn frog 0 0 0 health 10
``` 