Maintain relative asset paths when building

[raqbit]

Is your feature request related to a problem? Please describe.

Currently when you specify asset files, the files will be copied to the root of the build directory, even though they might have been in a sub-directory of the project. This means that the paths specified in the markdown (images, mostly) should unintuitively be relative tot the generated result html: ./foo.png instead of relative to the source markdown: ./media/foo.png.

Describe the solution you'd like

We should keep the directory structure for the asset paths by creating the parent folder(s) of all assets copied.

Describe alternatives you've considered

We could keep this "flatten" behavior but rewrite all mentioned urls in the markdown/html documents to the new relative file paths, but this would require hooking into the markdown-renderer and any potential custom urls (in markdown plugins) would probably not be detected.