Add feature that the ':only-child' pseudo class should work even though it's in ':not' pseudo class.

cite: [6.6.7. The negation pseudo-class](http://www.w3.org/TR/2011/REC-css3-selectors-20110929/#negation) in specification
> The negation pseudo-class, :not(X), is a functional notation taking a simple selector (excluding the negation pseudo-class itself) as an argument. It represents an element that is not represented by its argument.

Also contain a `:only-child` pseudo class.