Prompt for password if needed

[bombless]

Try to fix #1306
This cannot be merged now, since it don't hide password because I cannot find a cross-platform way to do it using std library or crates.io. (rpassword don't build on Windows because of the dependency to termios)

https://github.com/jeaye/ncurses-rs seems close to work on Windows, maybe we could make a new crate that handle password input through ncurses-rs. BTW, ncurses-rs isn't on crates.io (yet).

And after all, maybe the whole idea about prompting on cargo update is wrong.

So I make this PR mainly for help and revise.

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @alexcrichton (or someone else) soon.

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[alexcrichton]

Unfortunately hiding the typing of the password was why I did not implement this in the first place, and I think that it's a hard requirement for a feature such as this. In light of this I'm going to close this for now, but thanks for the proof-of-concept implementation!