Add ability to set theme for mail notifications

We already can set a different theme than the default one for our
Mailable. It would be very useful if there was a way to set the theme
for the mail notifications too.

src/Illuminate/Notifications/Channels/MailChannel.php

@@ -93,6 +93,10 @@ protected function buildView($message)
             return $message->view;
         }
 
+        if (property_exists($message, 'theme') && ! is_null($message->theme)) {
+            $this->markdown->theme($message->theme);
+        }
+
         return [
             'html' => $this->markdown->render($message->markdown, $message->data()),
             'text' => $this->markdown->renderText($message->markdown, $message->data()),

src/Illuminate/Notifications/Messages/MailMessage.php

@@ -31,6 +31,13 @@ class MailMessage extends SimpleMessage implements Renderable
      */
     public $markdown = 'notifications::email';
 
+    /**
+     * The current theme being used when generating emails.
+     *
+     * @var string|null
+     */
+    public $theme = 'default';
+
     /**
      * The "from" information for the message.
      *
@@ -134,6 +141,19 @@ public function template($template)
         return $this;
     }
 
+    /**
+     * Set the theme to use with the markdown template.
+     *
+     * @param  string  $theme
+     * @return $this
+     */
+    public function theme($theme)
+    {
+        $this->theme = $theme;
+
+        return $this;
+    }
+
     /**
      * Set the from address for the mail message.
      *