Allow embedding Spotify iframe#76

Besides soundcloud, it would be good to have alternative for embedding audio. But I don’t know what the consequence for it (DRM Stuff etc.), so yeah if this is not implemented it’s fine.

Example of iframe code:

<iframe style="border-radius:12px" src="https://open.spotify.com/embed/track/6L0uCWhXyBNncoDgfEvK1R?utm_source=generator" width="100%" height="152" frameBorder="0" allowfullscreen="" allow="autoplay; clipboard-write; encrypted-media; fullscreen; picture-in-picture" loading="lazy"></iframe>

I’ve whitelisted Spotify 👍