preliminary t84 support

Author: nerdralph

README.md

@@ -1,5 +1,5 @@
 # picoCore
-Small Arduino/Wiring core for 8-pin tiny AVRs: ATtiny13 and ATtiny25-85 series.  Arduino Blink sketch compiled for the ATtiny85 is 64 bytes and for the ATtiny13 is 54 bytes.
+Small Arduino/Wiring core for 8-pin tiny AVRs: ATtiny13 and ATtiny85 series, and preliminary support for the ATtiny84.  Arduino Blink sketch compiled for the ATtiny85 is 64 bytes and for the ATtiny13 is 54 bytes.
 
 I am writing this core from scratch, with code size and efficiency as the main goal.  I am making use of AVR assembler code when the code size and efficiency goals cannot be readily attained with C/C++.  Unlike the official AVR core, picoCore has compile-time checking of many arguments.  Calling digitalWrite(42) will cause a compile error with the message, "pin out of range".