Add .types() for command arguments

Command arguments are automatically assigned a type but there doesn't seem to be any support for explicit typing like with options. When entering in a phone number with special characters like so:

```
vorpal.command("send ")

app$ send +1212345678 "hello world"
```

the argument "number" is converted to a number, omitting the + symbol. Furthermore, I think the .types() function available for options should also support command arguments.