Pimroni's SHIMs (Shove Hardware in the Middle) are a new range of boards designed to be small, affordable, and handy. They're just 1mm thick (the bare board, not including mounted components) and designed to be soldered straight onto the GPIO pins of your Pi, if you wish, so that you can use them with HATs and pHATs on top.
Just plug your micro-USB power supply into the micro-B connector on OnOff SHIM, and then press the button once to switch on power and boot up your Pi. After installing our software (details below), you can press the button again to initiate a clean shutdown and completely cut the power to your Pi.
Pimroni put together a one-line-installer to install the clean shutdown daemon. It watches the state of BCM pin 17 and, when pulled low (pressed), it initiates a clean shutdown. Last thing, just before your Pi shuts down, BCM pin 4 is pulled low to completely cut power to your Pi.
To install the software, open a terminal and type
curl https://get.pimoroni.com/onoffshim | bash to run the one-line-installer.