Support loading config from env var or config file

[sbernauer]

Came out of #36 (comment):

Is this parameter [--additional-demo-files] necessary for all calls of stackablectl? Wouldn't be an environment variable or folder containing the custom demos also be a viable solution?

[sbernauer]

This topic came up again in #228. One solution could be to add a config_file.properties or config_file.yaml. Before implementing a look should be taken at e.g. curl and git.

[NickLarsenNZ]

We should use the XDG_CONFIG_HOME var (or fallback rules as defined in the XDG Base Directory Specification)

Eg: ~/.config/stackablectl/config.toml (or whatever the user has changed it to).