Fix for #112

I added a strict-mode flag to the ParserConfiguration, per gered's suggestion/thought. Doing so is consistent with Sensio's Twig implementation. Strict mode defaults to `true` in order to avoid unexpected behaviour for those unfamiliar with it.

Strict-mode off causes `{{ nonexistent }}` and `{% set nothing = null %}{{ nothing }}` to both output an empty string. Operations involving null or undefined variables still return their original exceptions.

Joao, I'm not sure if my implementation falls withing your accepted best practice for this project. Open to criticism.