If file is -, read from stdin

[onnozweers]

With view-token -, it will read from stdin, so you can copy & paste a token, then press ctrl+d to end the input, and the token should be decoded. Since the token is not provided on the command line, it can't be stolen by other users on the same system with the ps command. Convenient and still safe.

[hailihu]

I tested and it works as expected. However, I think using Ctrl+D to end input is not very intuitive. Most users (like I did), will try Enter first. Also, the current implementation shows the token in the terminal. I wonder can we make the implementation similar to get-macaroon? There, when a password needs to be provided to generate a macaroon, it is not visible.