Command.chain / Command.bind / Command.use

I'd like to request a simple feature that will make Vorpal a bit more composable.

```js
const pizzaOptions = vorpal =>
vorpal
.option('-p, --pepperoni', 'pepperoni topping')
.option('-o, --olive', 'olive topping')

vorpal
.command('pizza')
.use(pizzaOptions)
.action( /* ... */ )
```

It's effectively a monadic bind.