password helper: inherit stdin, stderr

should make fido2luks much easier to use in boot scripts since it will
allow for usage as follows:

`fido2luks open-token /dev/disk/by-uuid/sda1 test 'bash -c "read -p Pass PW 1>&2; echo $PW"'`

which will read the password from the current terminal

src/cli_args/config.rs

@@ -7,6 +7,7 @@ use std::fs::File;
 use std::io::Read;
 use std::path::PathBuf;
 use std::process::Command;
+use std::process::Stdio;
 use std::str::FromStr;
 
 #[derive(Debug, Clone, PartialEq)]
@@ -156,6 +157,8 @@ impl PasswordHelper {
                 let password = Command::new("sh")
                     .arg("-c")
                     .arg(&password_helper)
+                    .stdin(Stdio::inherit())
+                    .stderr(Stdio::inherit())
                     .output()
                     .map_err(|e| Fido2LuksError::AskPassError {
                         cause: error::AskPassError::IO(e),