key blocked after first command

Hi,

This npm package look just awesome but ... Even with the simplest code, showed at the main page:

```
const vorpal = require('vorpal')();

vorpal
.command('foo', 'Outputs "bar".')
.action(function(args, callback) {
this.log('bar');
callback();
});

vorpal
.delimiter('myapp$')
.show();
```

This doesn't work perfectly. Here is a [video](https://i.mlgimg.xyz/a666a5b3.mp4) that show the problem.

Alaanor.