support serialization to/from human-readable, text-based format

[timotheecour]

http://stackoverflow.com/questions/18873924/what-does-the-protobuf-text-format-look-like ref: https://developers.google.com/protocol-buffers/docs/reference/cpp/google.protobuf.text_format#TextFormat.Parser.WriteLocationsTo.details

I only see json and binary serialization (wire format)

the text format is more convenient than json and is commonly used would be nice to have it for compatibility