Add support for ATTINY84, ATTINY85, new function powerDownMoreTime and declare WEAK an ISR_RUTINE fucntion for WatchDog feature

[ortegafernando]

Add support for ATTINY84
Add support for ATTINY85
new function powerDownMoreTime for more than 8 seconds
declare a new function ISR_RUTINE WEAK to override in your sketch so you could use the watchdog feature. (see example for also a VERY LONG watchdog feature)

[ortegafernando]

Hi, please anybody to check this so rocketstream could merge it ? Thanks.
(for me it is working very well).

[wvdakker]

This one does not work for me

`Arduino: 1.8.7 (Linux), Board: "ATtiny24/44/84, ATtiny84, Internal 1 MHz"

Build options changed, rebuilding all
/home/user/Develop/sketchbook/libraries/Low-Power-master/LowPower.cpp: In member function 'void LowPowerClass::idle(period_t, adc_t, timer1_t, timer0_t, usi_t)':
/home/user/Develop/sketchbook/libraries/Low-Power-master/LowPower.cpp:651:4: error: 'WDTCR' was not declared in this scope
WDTCR |= (1 << WDIE);
^
/home/user/Develop/sketchbook/libraries/Low-Power-master/LowPower.cpp: In member function 'void LowPowerClass::powerDown(period_t, adc_t, bod_t)':
/home/user/Develop/sketchbook/libraries/Low-Power-master/LowPower.cpp:1032:4: error: 'WDTCR' was not declared in this scope
WDTCR |= (1 << WDIE);
^
exit status 1
Error compiling for board ATtiny24/44/84.

This report would have more information with
"Show verbose output during compilation"
option enabled in File -> Preferences.`

[juraj-ondruska]

Checked on ATtiny85, works perfectly