Correct the implementation `filter` and `is`

According to the jQuery API documentation on `$.fn.filter` [1] and
`$.fn.is` [2], 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`.

[1]: http://api.jquery.com/filter/
[2]: http://api.jquery.com/is/