Add support for authenticating with Consul

[ketzacoatl]

See here: https://www.consul.io/api#authentication

We can include the Consul Token for authentication by either adding the X-Consul-Token header or Bearer scheme Authorization header (Authorization: Bearer).

We'll need functions for accepting and updating the token to use with the consul client. We'll also want this token to be optional (though highly encouraged, it's not required).