cli: Refactoring command structure

A small needed refactoring, hopefully before CLI gains attention upstream.

* Refactor the command structure to match the Lua suggested module structure.
* Change "run" method name to "main".

Signed-off-by: Or Cohen <orc@fewbytes.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>
cli: Add "hello" boilerplate command example

Signed-off-by: Or Cohen <orc@fewbytes.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>