As a best practice, the main function should only be in charge of code that can be
easily verified by reading it. All the functionality should be done in different
functions: A configuration-parsing function and a run function that receives the
configuration is one way to keep code clean.
These functions should return a Result type so that the main function can handle the
error cases.
You can use the std::env module for dealing with environment variables.
The eprintln! macro will princt to stderr instead of stdout.