Add support for custom data when returning a list of all socket clients

[connected-yzhang]

We have a use case when a new client connects to a socket.io server, it would like to know all the clients that are in a particular room, and some metadata of the clients on the other side. So we implemented a custom hook function that enables the consumer of the 'clients' call on the adapter to implement custom callback to fetch client metadata in addition to the clientIDs.

[darrachequesne]

Hi! Could you please rebase against master (most notably after #146)?

How about adding a new requestType?

var requestTypes = {
 clients: 0,
 clientRooms: 1,
 allRooms: 2, // list of all rooms created on every node
 remoteJoin: 3, // make any socket.id join a given room
 remoteLeave: 4, // make any socket.id leave a given room
 customRequest: 5, // using your registered hook
};

[connected-yzhang]

@darrachequesne Hi thanks for the quick look-at. We're just trying to initiate a discussion to see if this is a change that you think is worth making, has a reasonable demand and is inline with where you envisioned the socket.io-redis adapter to go. If so, we're more than happy to rebase against the current master.