Allow mapping nested tenant properties to mail config (#20)

* Use data_get() to allow mapping nested tenant attributes to mail config

* Fix code style (php-cs-fixer)

* Test the data_get() change

* Improve code, add info to docblock

---------

Co-authored-by: PHP CS Fixer <phpcsfixer@example.com>

tests/Etc/mail_migrations/2019_08_08_000001_add_custom_column.php

@@ -0,0 +1,26 @@
+<?php
+
+declare(strict_types=1);
+
+use Illuminate\Database\Migrations\Migration;
+use Illuminate\Database\Schema\Blueprint;
+use Illuminate\Support\Facades\Schema;
+
+class AddCustomColumn extends Migration
+{
+    /**
+     * Run the migrations.
+     *
+     * @return void
+     */
+    public function up()
+    {
+        Schema::table('tenants', function (Blueprint $table) {
+            $table->json('mail')->nullable();
+        });
+    }
+
+    public function down()
+    {
+    }
+}