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`.