Version 3.0.0-alpha.1

First alpha release of Pico v3.0. This is a pre-release!

* [New] Kick-start development of Pico 3.0
* [Changed] Require PHP 7.0.8+
* [Changed] Update dependencies: Twig 2.12, Symfony YAML 3.4, Parsedown 1.7.4
            and Parsedown Extra 0.8.1; this is just an interim step, we'll
            update to Twig 3.0+ and Symfony YAML 5.0+ later