Add option to manually set serial port

the default is /dev/heliox, which is a symlink to /dev/ttyUSBx

src/main.rs

@@ -4,13 +4,16 @@ use std::time::Duration;
 #[derive(Parser, Debug)]
 #[command(author, version, about, long_about = None)]
 struct Args {
-    #[arg(short, long)]
     mode: String,
+    #[arg(short, long)]
+    port: Option<String>,
 }
 
 fn main() {
     let args = Args::parse();
-    let mut port = serialport::new("/dev/heliox", 115_200)
+    let default_address = "/dev/heliox".to_string();
+    let address = args.port.unwrap_or(default_address);
+    let mut port = serialport::new(address, 115_200)
         .timeout(Duration::from_millis(10))
         .open()
         .expect("Failed to open port");