Argument detected as option

I use `Vorpal` for a short time and recently found the following issue.

This is my code:
```js
const vorpal = require('vorpal')();

vorpal.command('add [text...]', '...')
.types({ string: ['text'] })
.action(function(args, cb){
console.log(args);
cb();
});

vorpal.execSync('add "-test"');
```
The output is the following:

```
Invalid option: 't'. Showing Help:

Usage: add [options] [text...]

...

Options:

--help output usage information
```

After some debugging I found out that https://github.com/dthree/vorpal/blob/master/lib/util.js#L22 is removing the quotes.