Add Pico::filterVariable() method
Add Pico::filterVariable() method

This method can be used to validate and filter input data and can be called via `Pico::getUrlParameter()` (URL GET parameters) and `Pico::getFormParameter()` (HTTP POST parameters). `Pico::filterVariable()` is basically a wrapper for PHP's `filter_var()` function with various compatibility extensions to allow theme developers to use its functionality in Twig templates. Therefore Pico 1.1 adds the `url_param` (`Pico::getUrlParameter()`) and `form_param` (`Pico::getFormParameter()`) Twig functions.

Resolves #305

TakamiChie/Pico,PhrozenByte/Pico,PhrozenByte/Pico,TakamiChie/Pico,picocms/Pico,TakamiChie/Pico,picocms/Pico,PhrozenByte/Pico,TakamiChie/Pico,picocms/Pico