apcupsd-service: add services.apcupsd.hooks option

Each attribute in this option should name an apcupsd event and the
string value it contains will be executed in a shell in response to that
event. See "man apccontrol" for the list of events and what they
represent.

Now it is easy to hook into the apcupsd event system:

  services.apcupsd.hooks = {
    onbattery  = ''# shell commands to run when the onbattery event is emitted'';
    doshutdown = ''# shell commands to notify that the computer is shutting down'';
  };