Add ability to add new functionality via plugins

[whyrusleeping]

With go1.8 we can now load plugins at runtime: https://golang.org/pkg/plugin/

I think we can do some really cool things with this. The main examples that stand out in my mind are:

• Add support for new ipld types
  • i've been struggling to decide how i want to ship the blockchain and git integrations, this solves the problem nicely
• Add support for new libp2p transports
  • I don't necessarily want to ship ipfs with support for utp, udt, webrtc, and friends by default. But if users want to try these out easily, using a plugin really makes sense to me here

[Kubuxu]

As a note it will work only on LInux right now.

[whyrusleeping]

I think we can close this now, we have plugins (At least where they are supported by go). @Kubuxu ?

[Kubuxu]

Yes. What we can do is open issue for plugin types we want to support.