Allow "preload" attribute

[bpevs]

Background

Basically, for blog posts with heavy media content, it's a large load for a visitor to immediately download a lot of media.

The preload attribute lets us do things like:

• <video preload="none" />: don't load until the visitor presses play
• <audio preload="metadata" />: only load metadata, and load the actual audio media when the visitor presses play

I worked around this for the heaviest files by adding media via iframe, but this isn't really optimal for all the audio examples I have in these posts.

Changes

• Adds "preload" attribute to the allow list.

NOTE: I didn't test this or clone to my computer or anything, and didn't look too hard at the repo, so I'm not sure if there are other requirements here.

[bpevs]

oh btw, I don't think I can add reviewers, but @sirodoht ^ No rush, just tagging because I don't trust github notifications

[sirodoht]

Hey! That looks good, thanks!

[sirodoht]

@bpevs It should be working, I looked at the audio element you have there and it includes the preload="metadata" attribute.

[bpevs]

Thanks! I see it as well!