Add PHP-CS-Fixer with the Symfony2 coding standards

[xiehan]

NPR used to have its own handcrafted PHP standards (briefly explained here), but last December we decided to adopt the Symfony2 coding standards and enforce them using PHP-CS-Fixer with a pre-commit hook in order to reduce the time spent during code reviews on pointing out standards violations. This practice has been very successful and is worth rolling out to our other codebases, particular open source ones like this one. It'd be great if we could similarly have our coding style enforced at commit time so that we don't even need to have a section on style in our CONTRIBUTING.md doc and can instead just assume that any commits to this codebase follow our standards.

[xiehan]

I've just added milestone v3.0.0 which will drop support for PHP 5.6 and 7.0 at the end of the year, so I think we should target PHP 7.1+ with this setup -- don't optimize for or bother to get it working with PHP 5.6 at this stage.