Cookbook: recipes for parse-command-line-arguments

[richardhuxton]

One example with Cmdliner, and two from the standard library (Sys.argv and Arg),

Happy to rework them if I've missed some details.

[sabine]

These recipes have two problems:

• cmdliner example focuses on one oddly-specific task and doesn't cover enough API surface (for comparison, look at https://rust-lang-nursery.github.io/rust-cookbook/cli/arguments.html#parse-command-line-arguments)
• stdlib example does not implement the same task as cmdliner example. A point of the cookbook is comparing the exact same task between different libraries / stdlib, so all the recipes for a task need to implement the same task. To facilitate this, it can help to add a description for the task, when it is unclear.

ETA: also, as a general remark, avoiding to open the modules in question does a lot to help people see where values are coming from (we do not have a LSP running here that would allow to inspect the code).