Allow setting configuration file path via environment variable #516

Hello and thank you for this incredibly useful service!

I'm currently trying to use iframely via docker-compose on a NixOS machine. One particular issue with NixOS is that the folders programs code is in are not writable, so (to my current understanding) I cannot change the config.<env>.js files that I'd need to configure iframely.

Would it be possible to allow passing a different config file to load at runtime via environment variables? I tried looking and this doesn't seem like a current feature. I would be open to submitting a PR for it if you support such a change.

Thank you!