Add a version string to all outgoing messages

[ssiloti]

BEP5 has been updated to document the long standing de-facto standard of including a version string in RPC messages. This is an important feature for identifying implementations which may be lacking features or misbehaving. The relevant section of BEP5:

A key v should be included in every message with a client version string. The string should be a two character client identifier registered in BEP20 followed by a two character version identifier.

For this implementation I would suggest using WebTorrent's client id of WW. Note that the convention is for the client identifier to identify the DHT implementation rather than the client application. Thus the same identifier should be used regardless of whether this module is being used by WebTorrent, peerflix, or some other client.

[feross]

@ssiloti Thanks for opening this issue. Agreed, we should fix this.

Does anyone want to take a stab at sending a PR for this?