Add a way to provide a default value when looking up a key in a Document

In many languages (including Swift) there is a way to provide a default value to use for a key that isn't present in a dictionary. 

The Swift-y way incorporates the value into the subscript getter:

let value = doc["x", default: 0]

See documentation here.

We should consider supporting this on Documents. 

Note we will need to think carefully about behavior when there are null values in documents.