[feature request] Implement command line help

[it-praktyk]

Usually Linux applications display help when the switch '--help' is used.

rpi-update start normal operations

root@raspberrypi:/home/pi# rpi-update --help
 *** Raspberry Pi firmware updater by Hexxeh, enhanced by AndrewS and Dom
 *** Performing self-update
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100 12762  100 12762    0     0  40454      0 --:--:-- --:--:-- --:--:-- 40514
 *** Relaunching after update
 *** Raspberry Pi firmware updater by Hexxeh, enhanced by AndrewS and Dom
 *** We're running for the first time
 *** Backing up files (this will take a few minutes)
 *** Backing up firmware
 *** Backing up modules 4.4.50-v7+
Invalid git hash specified

Please consider implement help. Thank you.