Allocate and expose per-instance pointers for plugins
Avoid use of uninitialized variable in print_battery_info

The boolean flag 'watt_as_unit' may be used without being initialized if the
configured battery path does not contain expected statistics (for example if
it is misconfigured and points to AC adapter info or simply an unrelated file).

Even though it does not cause ill effects, it causes a warning (true positive)
when running i3status under Valgrind.  Initialize the variable to make code
well-defined.