add attributes to image() as options

[CobusT]

Reference #231

This doesn't address all of the image options, but starts by addressing the most common requests... width, height, alt and title.

Example:

[CobusT]

Ooh, yes, style is a good one. I am not sure we want to just pass through all properties, though. Would like to hear some thoughts from @mbostock on that.

[mbostock]

I agree we should make props an argument and then simply Object.assign(image, props).