Working with offline content from Jake's tool for Carl

See #64 (compatability with Jake's tool)

Carl Malamud has a use case where he has downloaded, using Jake's tool, a bunch of items for storage and availability offline in India.

This item is a Meta task for all the challenges of working with it :-)

See the #carl branch of dweb-mirror for any code below, its not merged because its currently incomplete, mostly untested and adds about 10Mb of dependencies without currently adding functionality.

* [ ] Phase 1: download missing files
* [ ] Phase 2: Reading xml and zips instead of json