Add custom selector support

Axe-core makes fairly heavy use of CSS selectors in a variety of places. There are unfortunately lots of things that you can't do with CSS, that if we had access to them in axe-core would be quite beneficial. One way we could do that is to add custom selectors. Custom selectors is something that various CSS preprocessors support, so it isn't out of the ordinary.

Here are some ideas for what that might look like in axe:

```
::role("button") // Match all elements with the semantic role of button
::onScreen() // check if an element is visible in the page
::accName("") // Check if the element has an empty name
```