Bit of refactoring of help command to avoid NPE

Signed-off-by: Antoine Neveux <antoine.neveux@gmail.com>
Using i18n for list flows command

Signed-off-by: Antoine Neveux <antoine.neveux@gmail.com>
Removed list:flows command as now managed by the list command

Signed-off-by: Antoine Neveux <antoine.neveux@gmail.com>
Added an invoker allowing to execute commands programmatically

Signed-off-by: Antoine Neveux <antoine.neveux@gmail.com>