feat: media organization

[rochacbruno]

Use case:

I write a post with foo-bar as slug.

Banner discovery

If marmite finds a media/foo-bar.banner|card.png|jpg|.. it assumes the image as banner or card

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What is missing:

2. Media subfolder

If marmites finds a media/foo-bar/ directory it looks inside the directory for card|banner.ext

3. Having other files on the media/foo-bar/* is also a use case, user can do something like:

![](@/image.png)

Marmite will replace @/ with media/foo-bar/