Decode the json (see #51)

src/Resources/contao/forms/FormCalendarField.php

@@ -114,7 +114,7 @@ public function parse($arrAttributes=null)
     $this->disabledDays = $this->getActiveDisabledDays($dateFormat);
 
     // add the custom configuration
-    $this->customConfiguration = $this->dateCustomConfiguration;
+    $this->customConfiguration = htmlspecialchars_decode($this->dateCustomConfiguration);
 
     return parent::parse($arrAttributes);
   }