Autoescaping for string values

I made a autoescaping with escaping all string values when rendering. (original data does not affected and available inside template via `$this->get('name')` function.

This can be set as default behavior for Engine, or used directly when rendering a template.

Tested and documented as well. (Docs go in separate [PR#52](https://github.com/thephpleague/plates/pull/52))