Let user enter it if no PIN is provided

[xwjdsh]

I also do a little bit of change to the transfer command, just put the logic of displaying the QRcode before reading PIN.

Fixes #6

[Halimao]

user create --pin missing to hide password input and still return err "execute failed: flag needs an argument: --pin"

[xwjdsh]

Do not pass --pin and then it'll randomly assign a PIN value, this is a different case, the pin will assign to the new user. If we do update here, then it will break the behavior of "random PIN", although we can add a flag to let the user input a PIN specifically, it's unnecessary I think.