Nixt can detect and respond to interactive prompts

Adds the public API methods on() and respond() for matching and responding
to interactive prompts.

Inspired by https://github.com/jprichardson/node-suppose

```js
nixt()
.run(cmd)
.on('Your name: ').respond('Joe User\n')
.on(/email: /').respond('juser@example.com\n')
.end();
```

Closes #1