Add support for Consul unix sockets

[colebrumley]

This PR adds support for socket-based Consul connections. This would be especially convenient in overlay network environments where it could be difficult to tell whether you're talking to the local Consul node or a remote one. It'd also be useful in Docker-in-Docker use cases like RancherOS where you may want to run Registrator in system-docker so that it's available immediately on boot.

I piggy-backed onto the existing consul and consulkv Factories by adding a -unix suffix to the schemes and then doing a little additional parsing in New().

The service backend works as expected, but the KV backend looks a little funky. Since we can't rely on slashes for the path (and thus the KV prefix), there's an extra colon to delimit the prefix i.e. consulkv-unix:///var/run/consul.sock:/base/key.

Commits are squashed and docs are added, let me know what you think!

[progrium]

Seems like a weird special case, but also doesn't seem unreasonable.