Better image handling support

[DanH42]

Images currently need to be handled by plugins on a per-network basis. There are already weird glitches with Messenger's images, and additional networks will only complicate things more. There should be core support for sending images that gets handled by the network. Potential example:

reply({
    image: "http://example.com/image.jpg",
    alt: "Message to be displayed on text-only networks (if blank, use the image URL)"
});

Possibly also allow for supplying a message and an image in the same reply call, to be sent in the same message. Messenger allows this, and other networks could always just handle it on their end as two messages if needed.