`keybase sign` and plaintext password

When I run `keybase sign` with no options, I get asked for my private key password. When I start typing it, the passphrase shows up in plain text.

When I run `echo "asdf" | keybase sign`, everything works as expected.

So, maybe there should be an error if no piped stdin is detected on `keybase sign`?

Start with the `keybase sign` command entry point and reproduce both cases: interactive use with no options and piped input via `echo "asdf" | keybase sign`. The work is done when interactive signing no longer exposes the private-key passphrase and piped signing continues to work, with tests covering both behaviors.

Written by the indexing model from the issue text.