Optionally allow config to be passed as environmental variables

[wjziv]

Firstly, thanks for creating the project in the first place! I use the service almost daily.

I host locast2plex on a docker container through Unraid. There, it's simplest to pass configuration parameters through as environment variables, rather than supply a configuration file.

The small changes I supplied only seek out these environment variables if the config files does not exist. If no relevant environment variables can be found, it continues with the original failure method Else, this gathered information is written to config/config.ini and set up follows as normal.

Direct support on Unraid can be accomplished by creating a template for the environment variables/path mapping; I can handle this once I'm certain environment variable support is made the norm.

More Unraid support information here.