$.siblings now matches jQuery documentation and allows use of a selector

I ran into a situation where I needed to select siblings that matched a certain criteria, only to find that cheerio didn't support ```$.siblings('.selector')```, but jQuery core did. Looking at the tests for it, it appears it was *meant* to have supported it, yet the test selector ('li') returned identical results to using no selector.

This pull request implements the selector functionality and returns proper results in both cases. I tested using my function without a selector, my function with a selector, and the original function (no selector). The no-selectors returned identical results and the selector test returned filtered results.