Support part buffer for decode about msgpack-lite

[kawanet]

There is a public but undocumented lower-level class Decoder as I've mentioned at #31. I guess the code below would work with this class.

var Decoder = require("msgpack-lite").Decoder;
var decoder = new Decoder();
decoder.buffer = msgPackBuffer;
decoder.offset = 100;
var obj = decoder.fetch(); // decode the first element only

[KlonD90]

@kawanet thank you :) it is very helpful for me.