Add custom unmarshaling support

[merlinz01]

This PR adds the capability to define an UnmarshalMsgpack() method on any type, which allows customizing how unmarshaling works.

The UnmarshalMsgpack() method takes a single parameter of type any which gets the value which would be unmarshaled into an any type. The method should check the type of the parameter, which can be any type that msgpack supports. The method returns the value to assign and any error encountered.

One use case for this is illustrated in the tests provided, where a string composed of concatenated IP networks is received and the user wants to parse them into a slice of net.IPNet's on the fly.

[shamaton]

Hi.

Why don't you use ext format?
Do you know how to use it?
It seems like that we can encode the same as time.Time.

[merlinz01]

I suppose I could do it that way. The ext.Encoder and ext.Decoder interfaces are not documented; could you show me how to use them?

[shamaton]

Here is examples.
https://github.com/shamaton/msgpack/blob/master/time/decode.go
https://github.com/shamaton/msgpack/blob/master/time/encode.go

You implement your custom encoder and then call AddExtCoder when you start your service.

msgpack.AddExtCoder(encoder, decoder)

[merlinz01]

Even the examples aren't documented. You need to add comments in the code to explain what's happening and what the function is for. I don't understand how I should make it decode my type.