Option function and default parameter is not executed

In source diving the arguments passed to `command.option`, it appears that it accepts a 3rd and 4th parameter for defining a function and default value for the option, but it appears that these aren't actually being executed. In this test case, I was unable to get a default value or have the function execute. Am I doing something wrong, or is this just not supported yet?

``` js
#! /usr/bin/env node

let Vorpal = require('vorpal');
let cli = new Vorpal();

cli
.command('test')
.description('A test')
.option('--url ', 'Some url', function(val) { cli.log('VALUE!', value); }, 'http://some.url')
.action(function(args, done) {
this.log('URL:', args.options.url);
done();
});

cli.parse(process.argv);
```

``` sh
$ ./cli test
URL: undefined
```

``` sh
$ ./cli test --url other.url
URL: other.url
```