Add a DOM path to images that could be lazyloaded

[macbre]

20:50:23.153 lazyLoadableImages: analyzing which images can be lazy-loaded...
20:50:23.153 Spying disabled - analyzing which images can be lazy-loaded
20:50:23.154 lazyLoadableImages: 5 image(s) found, assuming 768px offset to be the fold
20:50:23.155 lazyLoadableImages: checking 3 unique image(s)
20:50:23.155 lazyLoadableImages: <http://0.0.0.0:8888/static/blank.gif> image (body > img#dot) is below the fold (at 900px)
20:50:23.156 lazyLoadableImages: <http://0.0.0.0:8888/static/example.svg> image (body > footer[2] > img[3]) is below the fold (at 1526px)

See #523