Page bundles

Each page can have extra content, like image files, video, audio, pdf, and even other markdown pages. It's available on Bundle pages (with filenames as index.md or _image.md).

It's a prerequisite to #26

• create bundle types: none, leaf, branch
• detect index.md (leaf) and _index.md (branch) as bundles
• create the page Resources, that will monitor extra files in the same folder
• scan and create the page resources list (in case of a branch, other pages in the same folder are considered as resources instead of independent pages)
• expose the list to the template
• build: copy all resources to the output folder
• serve: create a permalink for each resource and serve them
• create a resource search functionality on pages (i.e. "*.png")
• (optional) front matter definition for resources