Dynamic `blockquote.twitter-tweet` embeds
It would be great to also support Twitter's dynamic embedding mechanism in conjunction with e.g. libredirect.

Two examples of pages that currently don't work:

- https://www.reddit.com/live/18hnzysb1elcs/
  - Has many iframes to redditmedia.com, for example: https://www.redditmedia.com/mediaembed/liveupdate/18hnzysb1elcs/LiveUpdate_112dbf2c-aad3-11ec-b1dc-9e39912881e9/0
- https://mashable.com/article/best-funniest-tweets-week-february-18

Both of these pages seem to use a similar mechanism:

- A `blockquote` with `class="twitter-tweet"` is present in the page HTML, containing some placeholder text and a link (`a` tag) to the tweet
- The page loads Twitter's `widgets.js` script: `<script async src="https://platform.twitter.com/widgets.js" charset="utf-8"></script>`
- This script searches for `blockquote.twitter-tweet` and turns it into Twitter embeds (iframes).

If Twitter's `widgets.js` is allowed to execute, it does create iframes which libredirect successfully redirects to Nitter embeds. However, these iframes [remain hidden](https://github.com/libredirect/libredirect/issues/20#issuecomment-1076556969). I think `widgets.js` is waiting for the iframe to somehow signal that it finished loading, but that never happens in the Nitter implementation.

I can see one of two approaches to make this work:

- Make Nitter's iframes compatible with Twitter's `widgets.js` script
- Implement Nitter's own `widgets.js` script, which libredirect could load instead of the Twitter one.

Twitter's documentation for this mechanism:

- https://developer.twitter.com/en/docs/twitter-for-websites/javascript-api/guides/set-up-twitter-for-websites (how to add `widgets.js`)
- https://developer.twitter.com/en/docs/twitter-for-websites/embedded-tweets/overview (the "Render a Tweet with JavaScript" section - describes the `blockquote.twitter-tweet` convention)

Libredirect discussion: https://github.com/libredirect/libredirect/issues/20
