Allow to set URL to Faktory instance via FAKTORY_URL env variable

[Envek]

Context

In containerized world (especially in Kubernetes) it is very common pattern to pass secret values via environment variables without exposing them in command line arguments or elsewhere.

What's inside

Ability to specify URL to faktory instance (including password) via FAKTORY_URL environment variable.

Kubernetes usage

# Somewhere deep in StatefulSet declaration
        - name: metrics-exporter
          image: "praekeltfoundation/faktory_exporter:notyetreleased"
          imagePullPolicy: Always
          env:
            - name: FAKTORY_PASSWORD
              valueFrom:
                secretKeyRef:
                  name: faktory
                  key: password
            - name: FAKTORY_URL
              value: tcp://:$(FAKTORY_PASSWORD)@localhost:7419

[Envek]

By the way, @praekeltfoundation, @bearnard, do you want to send your current enhancements to the upstream repo or take over project maintenance? See lukasmalkmus#2

I would be happy to send some more minor pull requests, either to you or upstream.

Sorry, I can't create new issue in your fork so asking it here.

[lukasmalkmus]

Feel free to ping me if there is anything I can help with 👍

[Envek]

Merged into upstream in lukasmalkmus#4