Previews generation

It would be nice to make `render` subcommand capable of running in batch mode. At this moment, it can render preview for a single resource. If we could render previews for all resources in the folder, we could:
1. Test the code working with libpdfium in batch-mode.
2. Render previews/thumbnails on powerful server and allow light device to just sync them over the network.

Start with the existing `render` subcommand and trace how it currently renders a preview for one resource. Determine how resources in a folder are identified and how batch completion should be reported; done means previews or thumbnails are generated for all resources in that folder, including the libpdfium batch-mode test use case.

Written by the indexing model from the issue text.