implement globstar options for multiple extension matching

this was already in the spec, but now you can define different extensions that you want to clean.  

```js
hygienist('public', { clean: ['*.html', '*.json'] })
```

> note: we synchronously check for files to serve, so the more options extensions you define the more files we have to check for. they are checked in the order they are passed to `hygienist()`.