Add [-ext-contains="content"] attribute support to element hiding rules

[ameshkov]

It should work like :contains() pseudo-class which has not made to the CSS4 specification.

For instance, let we have an HTML like that:

<div class="red">
    In fact this element will be hidden because of the work "advert" inside.
</div>

This div will be hidden by this selector:
div.red[-ext-contains="advert"]

Please note, that contains will check the innerText property value, not innerHTML.