Extended metadata tags for embeds

As of now, Discord uses Twitter and OpenGraph metadatas to generate an embed when links are posted in chat. However, some websites like Twitter and YouTube gets treated differently and Discord generates custom embeds (for example Twitter embeds includes retweets/likes).

It'd be nice for website developers to be able to use the full potential of Discord embeds, as it'd be easier to put more data in them and give more context about what the page is about (for example if you're running a shop, you can add a "Price" field).

The syntax could be like