return object so method can be chained in builder

[nilslice]

17 should have included this, my mistake!

Enables method chaining to be used like so:

(new FFIMe\FFIMe("libextism.".soext()))
                ->include("extism.h")
                ->showWarnings(false)
                ->codeGen('Extism', 'Extism.php');

[ircmaxell]

Thanks!