Missing Timestamp extension type

[didactic-drunk]

https://github.com/msgpack/msgpack/blob/master/spec.md#formats-timestamp

I suggest emitting the ext timestamp format when serializing UTC Time rather than a custom format potentially solving issues with #58.

[kostya]

you can add custom time serialization for ext: example: https://github.com/crystal-community/msgpack-crystal/blob/master/examples/ext.cr, not sure it should be by default

[didactic-drunk]

I was surprised when my fractional seconds disappeared using the default packing method. I'm not sure if that's a bug or missing documentation but seems to violate the POLS, especially when .to_s doesn't show fractional seconds so Time's look the same when printing but comparisons fail.

Should there be a standard way to use MessagePack standard extension? Like:
@[MessagePack::Field(key: "my_key", timestamp: true)]
or a default name for the encoder?