Investigate the feasibility of adding flavor-like mechanism

[jangko]

As the usage of toml-serialization is growing beyond a mere config file parser, there is potential serializer clash between different subsystem using toml-serialization.

A lesson learned from json-serialization library, flavor-like mechanism is a good candidate to prevent such clash or contamination.

The granularity of the flavor should be configurable by user.
Backward compatibility is mandatory. toml-serialization stability, flexibility, and it's simplicity draw more users.