Use CSSselect library directly

@MatthewMueller I may be misinterpreting the goal of cheerio-select, but it seems unnecessary.

Commit message:

> The "cheerio-select" library does not offer additional functionality,
> serving as a simple wrapper around the "CSSselect" library. Its
> inclusion in Cheerio it makes the project:
> 
> - More difficult to understand (contributors must read through that
>   library's source before recognizing CSS selector bugs originate from
>   CSSselect)
> - Less efficient (it introduces an additional function call and
>   processing overhead)
> - More difficult to maintain (CSSselect releases must first be approved
>   by "cheerio-select" before they can reach Cheerio itself)