Add filesystem processor

[ikalnytskyi]

We see Holocron as a constructor to assemble a static site generator of your dream. In order to achieve this, we need to provide various primitives (processors) and do not tie users with hardcoded behavior.

One of such things is the source for created, updated and published attributes assigned to documents. So far source processors is responsible for assigning them. We probably want to see a standalone processor that would extract various important information out of filesystem.

Some attributes that could be extracted:

• created extracted from stats
• updated extracted from stats
• published extracted from file path (pass pattern for extraction)
• owner, group, permissions (see Preserve access permissions of documents #205)

Alternatively, we can end up with some date-in-filename processor that only cuts published attribute out of file path, and extract other filesystem attributes in source processor. It might be ok, since there's only one way bulletproof way to do it, while extracting published attribute might be tricky as various patterns might be used.