Correct the implementation `filter` and `is`

According to [the jQuery API documentation on `$.fn.filter`](http://api.jquery.com/filter/) and [`$.fn.is`](http://api.jquery.com/is/), these methods accept any of the following argument types:

- selector string
- function
- element
- jQuery object

Extend the `$.fn.filter` method to accept elements and Cheerio collections. This implicitly fixes the corresponding bug in `$.fn.is`.