Allow custom connection labels

[duksis]

Adds the possibility to configure custom labels to be added to the worker processes.

This can be done by adding labels keys to the connection config

def start(_, _) do
  children = [
    {FaktoryWorker, connection: [
      host: "localhost",
      password: "very-secret",
      labels: ["dev-machine", "elixir-#{System.version()}"]
    ]}
  ]

  Supervisor.start_link(children,
    strategy: :one_for_one,
    name: FaktoryWorkerUsage.Supervisor
  )
end

Resulting in the additional dev-machine label being displayed on the faktory worker dashboard

closes #191

[Ch4s3]

hey @duksis I just saw this. We don't do a ton of maintenance on this library, but I'm going to review this tomorrow and probably roll it into the next release. Thanks for the contribution, and sorry for the delay.