Add tare button

[mortenfyhn]

Add a physical tare button, and implement it in software. Should probably just be a simple momentary button. Even I can understand the circuit:

• [ ] Choose a button that fits and feels nice.
• [ ] Integrate it.

[mortenfyhn]

Also do #14 as part of this.

[mortenfyhn]

With internal pullup, no need for 3 wires and external resistor.

[mortenfyhn]

Closed by 229d202