Break Cache functionality into a separate class.

Create a TemplateCache class to hold all cache related functionality and settings. 

Set an instance of this class in the TemplateSeed class. Create a cache() method in TemplateSeed class to return the reference to the template cache instance so we can do:

```php
$tpl = new TemplateSeed(...);
if (!$tpl->cache()->isEnabled()) {
    $tpl->cache()->enableCaching();
}
```