wiring.c select which timer to use for millis() and other functions

Would be great to be able to define which timer to use for millis/micros/delay and may be others, where timer0 currently used. For example timer2 (where available) can be used instead.
This will be useful in case both timers 0 and 1 have to be clocked from external source to count pulses from two independent sources. Something like #define CORE_USE_TIMER2

Start in wiring.c and trace how timer0 currently supports millis(), micros(), and delay(). Determine the changes needed to select timer2 where available while preserving the existing behavior. Done means the core can configure an alternate timer for these functions without preventing timers 0 and 1 from counting external pulses.

Written by the indexing model from the issue text.