Prevent specific snippets from being cached

This allows users to prevent snippets from being cached. It can be turned on using `data-history-nocache` attribute on a per-snippet basis (works only with n:snippet macro though as of current implementation in Nette).

Use case: we navigate to a product page using AJAX and add the product to cart, invalidating a snippet with cart contents. Pressing Back, the cart snippet is reloaded from cache, losing its actual content.