warn: "options.generator" property is deprecated, use "output.name" instead

index.js#39L
```javascript
function isEbook(book) {
// 2.x
if (book.options && book.options.generator) {
return book.options.generator === 'ebook';
}

// 3.x
return book.output.name === 'ebook';
}
```

book.options.generator is a get/set,you could change to

```javascript
function isEbook(book) {
// 2.x
if (/^2+\.\d+\.\d+$/.test(book.gitbook.version)) {
return book.options.generator === 'ebook';
}
return book.output.name === 'ebook';
}
```