Sparse file structure support with virtual pages

[zeldigas]

Right now detection of pages in documents directory is only done when strict parent-child relationship is met. That means that parent pages can be either managed or we must skip unmanaged parent pages and put managed put to logically invalid directory.

Example of wiki structure

A <- managed
 - A.A <- managed
 - A.B
B <- unmanaged
 - B.A <- managed

For this structure, directory tree with files must be now

page_a.md
page_a/page_a.a.md
page_b.a.md //with custom parent metadata

This leads to mess in documents directory. Improve it in a way to have ability to put pages to relevant directory

Ideas:

1. Extra flag/parameter in config file to enable this mode --sparse/--no-sparse
2. For sparse scan mode we go deeper, with side effect any supported file in all directories are treated as pages
3. Would be also great to define "virtual" pages - to be able to put a link to it, but at the same time do not upload it so that it can be managed manually.