Support hybrid backend for web+native

[RangerMauve]

Similar to webtorrent-hybrid, it should be possible to load the SDK in a hybrid mode on Node.js where it connects to WebRTC in addition to raw hyperswarm to help seed browser networks.

This should make applications that use regular web browsers as a fallback more resilient and should make it easier to satisfy this use case.

Some thoughts:

• Will we need to wrap hyperswarm-web and hyperswarm in a hyperswarm-hybrid?
• Should node clients be using the proxy? (should probably avoid it, but it could be nice as a last resort instead of a TURN server)
• Can we do deduplication across the sources of peers?

Feedback / a PR / funding for this feature would be very much appreciated. :P