Improve libp2p config

[daviddias]

Right now, it is impossible to signal through the config if we want to use a transport just for dialing and not for listening, in fact, go-ipfs makes the hard decision of disabling any dialing from a transport from where we are not listening.

The issue is that with this design, we have to make something hacky/hard coded to enable the WebSockets transport within libp2p-swarm for dialing in the browser.

https://github.com/libp2p/js-libp2p/blob/master/src/index.js#L97-L102

Ideas:

• Have a dialer multiaddr addr to signal that no listener should be started, but it should be considered to dial anyway
• Have a more config option within the config.json that informs swarm to do that.

[daviddias]

Please review proposal here #166

[jacobheun]

With #166 merged I think we could add transport configuration for something like isDialer and then push all dialer only transports into the switch transports after startup. This could be enabled by default, as that is already a set expectation with web sockets.