Plugin::Core: fix non-existing export
AngelPS1::System: init at import time
Add experimental Battery plugin
Rename BatteryStatus to BatteryPercent
Prompt::Default: use BatteryStatus
BatteryPercent: do nothing if not supported in AngelPS1::System

Do nothing if the platform-dependent implementation of AngelPS1::System
does not have ->fetch_battery.
BatteryPercent: fix usage of fetch_battery result (charging / dis.)
Compiler: skip undefs
Compiler: update comments of reduce's work
MAJOR FIX: $/ was globally set to undef