Add new cosmetic procedural operator: :nth-ancestor(n)

The purpose of this new `:nth-ancestor(n)` operator is to
lookup the nth ancestor relative to the currently selected
node.

It is essentially equivalent to `:xpath(..)`, where
ancestor distance is expressed as a number rather than a
sequence of slash-separated `..`.

The rationale to introduce this new procedural selector
is to have a low overhead way to accomplish ancestor
selection.