Extend Message interface to allow embedded images to be extracted

[TheDiscordian]

Extend the Message interface (https://github.com/TheDiscordian/onebot/blob/master/onelib/onetype.go#L284) with an Images() []Image method. Image should be an interface that has methods for retrieving data, could be anything but for example:

type Image interface {
    Data() []byte // can only be empty if URL() isn't empty
    URL() string  // can be empty
}

[tippi-fifestarr]

great idea Boris!