Support a .toJSON method as part of the spec

[daviddias]

@wanderer and @kumavis have shown a lot of interested to have a conversation to JSON as part of the standardised interface.

My only concern is that although we can have a .toJSON, we really can't expect to have a fromJSON, otherwise we would be asking every single format implementor to spend a lifetime figuring out all the quirks of JSON, specially because it does mangle binary and numeric types.

Are we ok with having a .toJSON without a .fromJSON? Is that what people want?

[daviddias]

Following up on #11 (comment)