Support custom configuration path.

[voronind-com]

Issue: Currently you can only configure the app via yaml file inside the source files.

Desired: Allow to change the yaml configuration path i.e. with environment variable like HOMER_CONFIG=/path/to/config.yaml.

Context: I'd like to package this repo for the Nixpkgs repository, but it won't get accepted unless the app allows separation of sources and configuration.

[Rajamahal]

This seems to be especially problematic for use with TrueNAS scale. Difficult (impossible?) to edit the config.yml file due to the way scale now limits access to app source files.