* Use RList
* Support callback for null command (why?)
* Show help of commands
  - long commands not yet tested at all
  - added interface to export command list into an autocompletable
    argc, argv for dietline
* r_cmd must provide a nesting char table indexing for commands
  - this is already partially done
  - this is pretty similar to r_db
  - every module can register their own commands
  - commands can be listed like in a tree
