Allow the use of a custom parser

[darrachequesne]

The kind of change this PR does introduce

• a new feature

New behaviour

This would enable the use of some custom parsers, like the following based on msgpack:

const msgpack = require('notepack.io');
const Emitter = require('component-emitter');

class Encoder {
  encode(obj, callback){
    var encoded = msgpack.encode(obj);
    callback([encoded]);
  }
}

class Decoder extends Emitter {
  add(obj){
    var decoded = msgpack.decode(obj);
    this.emit('decoded', decoded);
  }
  destroy(){}
}

exports.Encoder = Encoder;
exports.Decoder = Decoder;

And then:

const server = require('http').createServer();
const myCustomParser = require('./my-custom-parser');
const io = require('socket.io')(server, {
  parser: myCustomParser
});

That would also provide a workaround for socketio/socket.io-parser#61.

Related: #1652 #1946

Note: that would need to update socket.io-client and socket.io-adapter too.