Add echo command

src/main.rs

@@ -16,6 +16,9 @@ fn main() {
     let args = parts;
 
     match command {
+        "echo" => {
+            println!("{}", args.peekable().peek().map_or("/", |x| *x));
+        },
         "cd" => {
             let new_dir = args.peekable().peek().map_or("/", |x| *x);
             let root = Path::new(new_dir);