fix: status mesages go to stderr not stdout

Progress spinner and status line are written to stderr
instead of stdout.

No longer any need to detect if stdout is being redirected.

tests/test_flags.rs

@@ -10,6 +10,9 @@ use std::str;
 
 fn build_command<T: AsRef<OsStr>>(command_args: Vec<T>) -> String {
     let mut cmd = &mut Command::cargo_bin("dust").unwrap();
+    // Hide progress bar
+    cmd = cmd.arg("-P");
+
     for p in command_args {
         cmd = cmd.arg(p);
     }