Provide flag to import metadata about dangling blobs

Context

In !136 (merged) we have introduced a tool to import metadata from a registry filesystem into a database.

Problem

Dangling blobs are not imported, only referenced ones. This means that the import process also acts as a garbage collection. This can be seen as a feature, but we should provide a flag to import dangling blobs as well if wanted.

Solution

Provide a new command flag (-b) to import dandling blobs.