Lazy loading of commands

[janluke]

Inspired by

• How would I use lazy loading with cloup? #102

In this answer, I propose a possible solution:

Give a look to this example: https://github.com/janluke/cloup/tree/lazy-loading-example/examples/lazy_loading.

The idea is that rather than extending a MultiCommand, I define proxies (LazyCommand and LazyGroup) for lazy-loaded commands.
Then I use those lazy commands as if they were normal commands.

Nonetheless, I've never used lazy loading in my programs, so I'd appreciate feedback on this. Also, if you are interested in this feature, please let me know with a thumb up.