Update watch.py
Update watch.py

-Changed error handling for invalid commands and incorrect script arguments
-Added optional diff argument to display change in numerical values across command executions
-Changed argument structure to the following:
optional arguments:
  -h, --help            show this help message and exit
  -c COUNT, --count COUNT
                        Number of times to issue the command (default 3)
  -i INTERVAL, --interval INTERVAL
                        Wait time between command iterations (default 5 sec)
  -d, --diff            If numerical values have changed in an ouput print
                        difference between previous and current command
                        iteration
extremenetworks/xkit,extremenetworks/xkit,extremenetworks/xkit,extremenetworks/xkit