refactor: strict types, typed props, split toXml/generate, custom exception

src/Exceptions/MissingRequiredFieldException.php

@@ -0,0 +1,9 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Wearepixel\LaravelFacebookFeed\Exceptions;
+
+use RuntimeException;
+
+class MissingRequiredFieldException extends RuntimeException {}