Add implementation for Consul

[philippgille]

Consul is probably mostly known for being a service registry in a microservice deployment, but it's explicitly advertised as key-value store as well. And because it's one of the most popular service registries this means it's already running in a lot of deployments and software developers might prefer to use their existing infrastructure instead of having to set up something new like Redis.

So gokv should add an implementation for Consul as key-value store.

• https://github.com/hashicorp/consul
• https://www.consul.io