Allow using default configuration file

src/main.rs

@@ -105,7 +105,13 @@ fn process_package(packages_path: &String, package_name: &String, url: &String)
 }
 
 fn get_configuration_file_path() -> String {
-    args().nth(1).expect("Required parameter configuration file missing.")
+    let arg = args().nth(1);
+
+    if arg.is_none() {
+        println!("Using default configuration file: aur_config.toml")
+    }
+
+    arg.unwrap_or("aur_config.toml".to_string())
 }
 
 fn get_configuration_file_contents() -> std::io::Result<String> {