Add DTLS protocol

[backkem]

We're considering building DTLS transports in libp2p/libp2p#49. Therefore, I suggest to add a 'dtls' multiaddr protocol. I'd like to have some input on the following:

1. We don't really need a 'value' part of the multiaddr to establish a connection. However, we could add a certificate fingerprint as a form of authentication. Is this a good use of the 'value'? Can this be made optional?
2. Should I open a PR on this, the multicodec or both repo's?