API: Shape the list of zettel metadata

In most cases, it is not essential to list all zettel. Typically, you are interested only in a subset of the zettel maintained by your Zettelstore. This is done by adding some query parameters to the general GET /j request.

• Select just some zettel, based on metadata.
• Only a specific amount of zettel will be selected by specifying a length and/or an offset.
• Searching for specific content, not just the metadata, is another way of selecting some zettel.
• The resulting list can be sorted according to various criteria.