Special thanks to Spence Konde for making the Micronucleus bootloader a standard option in his ATtiny core. Follow these instructions to add this core to the Arduino IDE:
After a successful install you will find the new boards under Tools -> Boards, and select ATtiny84a (Micronucleus / California STEAM).
You might have to install the Micronucleus drivers manually. See for more information the Quick Gotcha list: