Inconsistencies when reading 0 as argument

[AljoschaMeyer]

The following code outputs 0 for the input t -index 0, but outputs undefined for the input t -i 0.

var Vorpal = require('./../../lib/vorpal');

var vorpal = new Vorpal();

vorpal.command('t')
.option("-i, --index <index>")
.action(function (args, cb) {
  console.log(args.options.index);
  cb()
});

vorpal.delimiter('foo:').show();

[dthree]

Fixed.