Add macroable trait to FcmMessage class

[gdebrauwer]

In all my fcm notifications, I create an FcmMessage object with the same AndroidConfig and ApnsConfig. It would be nice if I could create a macro that makes it possible to hide that shared code.

return FcmMessage::create()
    ->default() // macro that sets android and apns config
    ->...

// Instead of having to duplicate those configs in every notification
return FcmMessage::create()
    ->...
    ->setAndroid(
        AndroidConfig::create()->...
    )->setApns(
        ApnsConfig::create()->...
    );

[dwightwatson]

Nice one, thanks!