Handle official (MC| - prefixed) Custom Payload packets

Minecraft defines a couple of packets through the Custom Payload mechanism. Currently, node-mc-proto decodes every custom payload as a simple byte array, and API users are left to decode those themselves. This issues proposes two things

1. Add a new mechanism for API users to write their own custom payload decoder.
2. Add a custom payload decoder for all the official channels.

We might want to support and manage the "Register/Unregister" channel automagically for this.

In 1.8, the official channels (Ignoring the custom channel management ones) are :

MC|AdvCdm

Name	Type	Description
Mode	Byte
X	Int	If Mode == 0
Y	Int	If Mode == 0
Z	Int	If Mode == 0
EntityID	Int	If Mode == 1
Command	String

MC|Beacon

Name	Type	Description
Effect1	Int
Effect2	Int

MC|BEdit

Name	Type	Description
Book	Slot?

MC|BSign

Name	Type	Description
Book	Slot ?

MC|BOpen

Name	Type	Description

MC|ItemName

Name	Type	Description
ItemName	String

MC|RPack

(Deprecated, use Resource Pack Send (Play, 0x48, clientbound) and Resource Pack Status (Play, 0x19, serverbound) packets)

Name	Type	Description
URL	String

MC|TrList

Name	Type	Description
???

MC|TrSel

Name	Type	Description
Slot ID	Int	The selected slot id of the trading inventory

MC|PingHost

(Deprecated in 1.7)

Name	Type	Description
???

MC|Brand

Name	Type	Description
Implementation	String

Start by locating the custom payload packet handling and the API surface for registering decoders. Compare that path with the listed official MC| channels and their field formats; done means users can supply custom decoders and the official channels are decoded without manual byte-array parsing.