feature: Support for dereference links -L follow

du has -L flag which allows it to dereference or follow
symlinks. Clone this feature into dust.
https://github.com/bootandy/dust/issues/276

src/cli.rs

@@ -38,8 +38,16 @@ pub fn build_cli() -> Command<'static> {
             Arg::new("ignore_links")
                 .short('l')
                 .long("ignore-links")
+                .conflicts_with("dereference_links")
                 .help("Ignore links"),
         )
+        .arg(
+            Arg::new("dereference_links")
+                .short('L')
+                .long("dereference-links")
+                .conflicts_with("ignore_links")
+                .help("dereference sym links - Treat sym links as directories and go into them"),
+        )
         .arg(
             Arg::new("limit_filesystem")
                 .short('x')