Add placeholder, blurred versions of images

[NathanLovato]

Images now use the browser-native lazy loading feature: they only download when reaching the image on the page. The issue is the placeholder elements have a height of 0 and pop into the page, pushing the text down.

We should encode some base64 image in the html as a placholder. Something blurry as on medium could be nice, ~8 to 16 pixels wide.

Here's a snippet to use hugo's image render processing feature to generate a preview image: https://github.com/gj52/HugoSample/blob/master/themes/photon/layouts/_default/_markup/render-image.html