Add clone() methods to Document, Directives, Schema and all Nodes

[eemeli]

Fixes #130

This adds a deep-copying clone() method to ~everything.

It should also work pretty much automatically for custom nodes, unless they're doing something really exotic that the Object.create calls don't catch.

I also considered static from() methods and copy constructors as alternatives, but using clone() seemed to fit best with the node content structures, and by extension it's easier to then use the same API shape everywhere.

CC @gbouthenot, @christopherthielen