Use SignalStrength for format icons

[rbnis]

This adds the ability to set format icons similar to the pulseaudio module.

A config for this could look like this (Needs FontAwesome Pro):

"format": "{icon}",
"format-icons": {
  "wifi": ["", "" ,""],
  "ethernet": [""],
  "disconnected": [""] 
}

Also if fixes getIcon() in ALabel.

[Alexays]

Thanks!