Support environment variables in config file

[BSFishy]

Allows yaml files to include !ENV tags so that environment variables can be passed through to the config. E.g.

instances:
  sonarr:
    - base_url: "http://sonarr.media"
      api_key: !ENV SONARR_API_KEY

  radarr:
    - base_url: "http://radarr.media"
      api_key: !ENV RADARR_API_KEY

Fixes #294

[ManiMatter]

Thank you for the contribution.
Would it make sense to mention this feature in the readme?

[BSFishy]

Updated readme :) let me know if the wording looks good

[ManiMatter]

I think it‘s good (deleted my previous comnent), thanks a lot for the contribution