Enabling a custom decoding strategy for FirestoreDecoder *HACK*

[dannypier]

This enables an example usage as such

let dateDecoding = FirebaseDecoder.DateDecodingStrategy.custom({ (decoder, value) -> Date in
                        guard let timestamp =  value as? Timestamp else {
                            return Date.distantPast
                        }
                        return timestamp.dateValue()
                    })
let thing = try FirestoreDecoder().decode(Thing.self, from: document.data(), dateDecoding: dateDecoding)

[alickbass]

TBH, I think that it is indeed a hack and the proper way would be jus to make Timestamp conform to Codable. Look to this PR #43

[dannypier]

Totally agree.