eww: delay/defer image loading

[T. V. Raman]

If I use eww to display rss feeds that are converted to html via
xslt I notice that eww is busily loading lots of urls in the
background -- many of these images.

Example: Cnet Tech News RSS:
http://feeds.feedburner.com/cnet/tcoc

I got a number of these downloads to not happen by temporarily
advising eww-display-image to be a no-op -- could we have a
user-customizable option to defer loading images?
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