implement the "windows newline" trick

because on windows we add `\r\n` and only `\n` on linux, we can
just conditionally add `\r` when on windows and then always add
`\n`.

src/formatting.rs

@@ -149,10 +149,9 @@ pub fn format_inner(contents: &[u8], _config: &Config) -> Vec<u8> {
 /// It is used frequently in `nufmt`, so
 /// we have a wrapper to improve readability of the code.
 fn insert_newline(mut bytes: Vec<u8>) -> Vec<u8> {
- // If I need cfg windows, then I need \r\n
- // let newline = vec![b'\r', b'\n'];
- let newline = vec![b'\n'];
- bytes.extend(newline.iter());
+ #[cfg(windows)]
+ bytes.extend(b"\r");
+ bytes.extend(b"\n");
  bytes
 }