feat(layouts): lazyload images with lazysizes

[ghost]

dramatically reduces page load time, jank and image data consumption

fixes #30, relates to gohugoio/hugoThemes#244

[bep]

@digitalcraftsman might have more insight, but we already have a lazy loader:

b0a25ab

Not saying which one is better, we cannot have two!

[ghost]

@bep It does not work. I will remove unveil.

[bep]

It works fine for me in Chrome.

[ghost]

It works fine for me in Chrome.

It doesn't work for me in Chrome, nor does it work for WebPageTest. Please see the metrics I've provided. I've removed the previous loader, which is subpar because:

1. It uses jQuery and has to wait for the entire page to parse, and jQuery and all other jQuery dependencies to load, as configured, before activating
2. It should not be unsetting the src attribute of images. Lazysizes properly uses those for LQIP, and whenever a src attribute is provided a browser will try and load them if the script isn't fired immediately (which may explain why it works on your machine).

[ghost]

Note defer here is not necessary but has been added as a convenience for older versions of Internet Explorer, which do not make use of the async attribute.

[bep]

OK; let us test it.

[ghost]

Thanks for the merge. Here are the updated metrics:
http://speed.habd.as/themes_gohugo_io-cable/?period=day