Add number type to options for input validation purposes

[jacobdeichert]

The port option below has type: number which would allow us to validate the input is correct.

Note that even though we can at least validate it, it still gets turned into an environment variable which is just a string. The benefit here is that the script can assume it's always a number and cast it as such.

## serve

**OPTIONS:**
* port
    * flags: -p --port
    * type: number
    * desc: Which port to serve on

~~~node
const port = Number(process.env.port);
console.log(port * 2)
~~~

[lidongjies]

Can type be a URL or a regular expression?

[jacobdeichert]

We can definitely add other types for validation purposes. type: number would validate the input is a number... so type: url could validate the input is a correctly formatted url and type: path could also be handy.

Allowing type: /^some_regex$/ could be powerful but might be overkill if we can get away with mostly high-level types 👍