main: Accept addresses as command-line arguments

Now that the `MacAddress` struct can handle parsing arbitrary input
without panicking, the main entry point can accept values from the user
in the form of command-line arguments. Users can pass one or more
addresses as positional arguments to the `wakeonlan` program, and a
magic packet will be sent to each one in series.

src/main.rs

@@ -1,9 +1,22 @@
+use std::env;
+
 mod mac;
 mod magic;
 
 
 fn main() {
-    let mac = mac::MacAddress::from_string("0c:9d:92:0e:3a:41").unwrap();
+    let mut args = env::args();
+    args.next();
+    for arg in args {
+        let mac = match mac::MacAddress::from_string(&arg) {
+            Ok(m) => m,
+            Err(e) => {
+                eprintln!("Invalid address: {} ({})", &arg, e);
+                continue;
+            },
+        };
         let packet = magic::MagicPacket::new(&mac);
+        println!("Sending magic packet to {}", mac);
         packet.send().unwrap();
+    }
 }