specify initial schema in the dat constructor

[mafintosh]

there should be a way to pass an initial protobuf schema to dat that it will use as the baseline.

for storing something like https://github.com/openaddresses/openaddresses/blob/master/sources/us-oh-erie.json that uses a predefined format it will allow dat to save (a bit of cpu) and have better compression on disk (since we can specify the nested protobuf types) etc.