Add @dynamicMemberLookup to Document

Swift 4.2 introduces the @dynamicMemberLookup attribute. Basically, this is just syntactic sugar, allowing you to use dot notation to look up arbitrary properties on a type.

A natural place for us to use this is on documents. This will allow users to do things like the following:

var doc: Document = ["a": 1, "b": 2]

print(doc.a) // prints 1

print(doc.x) // prints nil

print(doc.b) // prints 2

Implementing this will require adding the attribute to Document, and adding a new subscript(dynamicMember: ) method that behaves identically to our existing subscript getter.