user needs to add selectors crate to Cargo.toml in order to use add_class

[ctm]

This is a super low priority nit.

I added scraper to a little tool I've written and found that in order to use Element::has_class, I need to add the selectors crate to my Cargo.toml file and I need to pin it to the version of selectors that scraper is using.

I had to do that because nothing re-exports CaseSensitivity.

Adding pub to the use line in src/node.rs:

use selectors::attr::CaseSensitivity;

i.e.,

pub use selectors::attr::CaseSensitivity;

allows me to remove selectors from Cargo.toml and just use scraper::node::CaseSensitivity.

It may be that exposing CaseSensitivity somewhere other than the node module makes more sense.