Adjust JS `Image` sizing properties to be more inline with standard.

[mpaperno]

• allow specifying dimensions in constructor as arguments (new image(w,h)) or in options object;
• the width and height properties are now settable, and return the set size, if any, otherwise the actual image dimensions;
• added naturalWidth & naturalHeight read-only properties which always return actual image dimensions;
• added optional options argument to loadImage() which gets passed to Image constructor;
• updated TypeScript definitions and no longer inherit from global Image (which is an HTMLElement with many things our version is missing);
• added tests.

[samizdatco]

I'm going to merge this just to make it possible to rebase the SVG PR. Happy to pick up the image-init syntax question again once I've dug into the SVG stuff a bit.