Added customizable poweroff command

[miohna]

Adds an option to configure the poweroff command executed when
triggering a shutdown. Poweroff command to use is read from file
settings/Poweroff_Command, default sudo poweroff.

Scenario:

• You are powering you Pi via an extension board, e.g. MoPi 2,
  that cannot detect a regular poweroff and stays powered-on itself. Via a
  customizable poweroff command you can trigger the extension board to
  power off the Pi in the prefered way which finally also turns-off the
  extension board.
• You like to add some extras, e.g. LED illumination, on power off. A
  customizable poweroff command provides the "user" a nice hook for whatever
  stuff he likes to add before the actual power off :-)

[Huseriato]

Cool idea. I checked you pull request. Looks fine. :)

[MiczFlor]

HI @miohna
could you add this to the wiki, please? Ideally with other options? I think it should go into this section:
https://github.com/MiczFlor/RPi-Jukebox-RFID/wiki/MANUAL#other-relevant-files-in-the-settings-folder
And: IF there are a limited number of options, could you add them here in the thread? I will then make them part of the WebUI configuration page.
Thanks!

[miohna]

Wiki is updated: https://github.com/MiczFlor/RPi-Jukebox-RFID/wiki/MANUAL#settingspoweroff_command

My use case is the MoPi2 board for which I need to power off the RPi via sudo mopicli -wsd 1 to get the board integrated smoothly. It could be a switch in the WebUI like for the RFID Reader, but I do not think that so many people currently using the MoPi2 as power interface for the Phoniebox. Therefore, it is probably not worth it to change the WebUI.

In general, I am sure there are many other options to use this feature for fun stuff.

[MiczFlor]

Thanks @miohna , you are the best!