Add support for --automatic.

src/main.rs

@@ -1,4 +1,4 @@
-use std::env;
+use std::{env, process};
 
 fn main() {
     let args: Vec<String> = env::args().collect();
@@ -6,8 +6,19 @@ fn main() {
         // first arg is executable
         swayout::print_layout_names();
     } else if args.len() == 2 {
-        swayout::apply_layout(&args[1]);
+        let arg = &args[1];
+        if arg == "--automatic" {
+            if let Some(layout_name) = swayout::apply_automatic() {
+                println!("{}", layout_name);
+            } else {
+                eprintln!("no automatic layout available");
+                process::exit(2)
+            }
+        } else {
+            swayout::apply_layout(&args[1]);
+        }
     } else {
-        panic!("Usage: {} [layout]", args[0].as_str());
+        eprintln!("Usage: {} [layout]", args[0].as_str());
+        process::exit(1)
     }
 }