Add data-preload attribute for iframes

[maxrothman]

Allows lazy-loaded (i.e. data-src) iframes to be preloaded when they
come within the viewDistance, rather than once they're visible.

Replaces #2353, which was against master rather than dev.

[maxrothman]

A couple of interesting things I noticed while working on this change:

• <img>s aren't unloaded when they leave the viewDistance. I assume this is intentional, since the cost of downloading them has already been paid?
• When preloaded iframes are loaded, the src attribute replaces the data-src attribute. For non-preloaded iframes (i.e. the previous behavior), the src attribute is added in addition to the data-src attribute. Was this intentional? Is it worth the effort to make them behave the same way?

[chreds]

This is great! Thanks for the commit. Should the console.log() be removed from line 3718?

[maxrothman]

Yes, whoops 😳

[maxrothman]

Pinging @hakimel who reviewed a previous iteration of this PR

[hakimel]

Looks great! Will find time to review and test this soon.

No need to ping me btw – I receive notifications for everything in this repo :)

[hakimel]

This has been merged now!

I added some tests and made a few smaller changes in d6f0f41

[maxrothman]

Awesome! Any sense of when the next release is going out?

[hakimel]

3.8.0 was just released and includes this update.

[maxrothman]