Convert to command line application

Hey Onorio,

You'll have to test this for me, but this makes your initial script into a CLI application. You can build the executable from the root of the project with `mix escriptize`. You can then execute it using `./firewall_utility`. This can then be executed from powershell or a batch script.

I've also made a number of style changes, which I'll attempt to list below, but if you notice anything I missed, or want to discuss any of these points, feel free to let me know.

- Idiomatic Elixir uses 2 space indentation
- Likewise with putting public functions before private in the file
- You can build strings that are easier to read using `"Hello #{name}"` vs `"Hello " <> name`
- Use pipes to convert nested function calls to a more natural reading style (left to right).
- Use `unless` instead of `if not`

And that's about it. Let me know if you hit any snags with these changes!