Cache the image of the poster attribute of the video element

[PunchlY]

• [x] I have read this document: https://miniflux.app/opinionated.html#feature-request

I have an article like this:

<video poster="https://example.com/img.png" src="https://example.com/video.mp4" controls="" data-fullscreen-container="true" data-popup-wrap-id="h5player-popup-wrap-jjz3q8tskwk"></video>

I found that the poster attribute will not be proxied like this /proxy/******/******.