What kind of external power supply can I use through usb? Can I use powerbank?
INPUT :100-240V, 50-60Hz --- ???
OUTPUT: 5V, 2A --- ???
Yeah, any power supply which connects to USB should work. USB has it's own voltage standard, and the device itself is current limited so anything should work.
We actually discussed at length whether or not to include a wall plug. I personally don't like getting something which doesn't have all of the pieces needed for normal operation... but at the same time, we have people buying the device from many many different countries with many different plugs. As a small dev team, we don't really have the time to provide the correct outlet types for everyone, so we felt that just sending the USB would be the best option.