Support common data output format

[doyubkim]

I've been asked about any possibility of using HDF5 or similar kinds of data format. While Jet is using FlatBuffers for its serialization solution, supporting commonly sharable/viewable data format also seems to make sense. However, I do not want to introduce too many library dependencies to jet since it is mainly for the book. If I can find a better solution for this (something like a plug-in model), I will consider integrating such formats.