Creating a program interface with the flag package

The previous recipe describes how to access the program arguments by a very generic approach.

This recipe will provide a way of defining an interface via the program flags. This approach dominates systems based on GNU/Linux, BSD, and macOS.  The example of the program call could be ls -l which will, on *NIX systems, list the files in a current directory. 

The Go package for flag handling does not support flag combining like ls -ll, where there are multiple flags after a single dash. Each flag must be separate. The Go flag package also does not differentiate between long options and short ones. Finally, -flag and --flag are equivalent.