- adds a constructor which allows configuring db path
  - adds pieces(1) and subcommands
  - allows for adding an address and linking between 2 addresses

This is still super raw and I'm undecided about various things.

- Moves more stuff into App::MarkFiles.
  - Renames each() to each_path() and simplifies the interface from
    handing a hashref to the callback to just giving it a path string.
  - Adds mark-remove(1) and renames App::MarkFiles::unmark() to remove()