Add dtls multiaddr

[backkem]

This PR adds a dtls multiaddr to resolve #75. I've not foreseen a value part for now. 0x20 also seems free in multiformats/multicodec. Let me know if I should also open a PR on that repo.

[Stebalien]

So, regardless of whether or not go-libp2p can currently use dtls, we can still add the protocol code. However,

1. We should first PR against https://github.com/multiformats/multicodec as that's the complete list of codecs.
2. We should probably pick a two-byte multicodec as we have very few single-byte multicodecs available.