Add a `Source` for directory that uses files as keys

[ludat]

Create a new Source that takes a directory path (possibly extracted from a Config and to get Key you simply look into the file with that key name.

Not sure what should we do on some special cases like:

• Many filenames correspond to the same key, which one should we use?
• Listing should normalize the filenames somehow to correspond with the first behavior
• Should we allow file extensions and strip them? what about overlaps? or just use the canonical key as filename?