Add parameter and return types, and consider using strict type mode

[nicolus]

See #24

PicoFeed now required PHP 7.1, but it's mostly still a PHP 5 library under the hood. Adding parameter and return type specifications and using strict mode throughout would help avoid problems like happened with #22, and would also help formalize the API.

[nicolus]

I'm all for adding Parameters and return types whenever possible. I'm not 100% sold on the whole strict types thing, but I can't argue that it would have prevented some bugs, so yeah that would probably be a good idea.