Upgrading `commands` module to package, introducing new modules
`handlers` and `args`:

- Handlers.py contains the command handler functions written so far.

- Arg.py makes it possible to register command argument placeholders,
  similar to registering commands, so that we can now apply custom
  autocomplete results and input value validation for each argument
  type in use.

TODO: commands register method is still to be taught to register the
arguments coming with newly registered commands.