Improve Configuration

[viral32111]

The configuration should be extended to file and environment variables to conform to most scenarios, using the following order of precedence:

1. Command-line flags (e.g., --address <string>)
2. Environment variables (e.g., KASA_SMART_PLUG_ADDRESS=<string>)
3. File (e.g., address = "<string>")

The configuration file should be something like INI as it provides comments and has clearly identified sections with no worries of indentation requirements like YAML. Definitely don't use JSON as that isn't very human friendly to edit. For example:

[plug]
address = 192.168.0.5
port = 9999

[metrics]
path = "/"

The configuration file should be self-documenting too, including plenty of comments and a link at the beginning to any online documentation (GitHub Wiki?).