Integrate Kiwix with zsync

PROBLEM
Many .zim files, eg for Wikipedia, are huge. If you wish to update your wikipedia zim, you must re-download everything. This takes time, uses lots of bandwidth, means that new torrents need to be started frequently, and fragements people seeding torrents across versions.

PROPOSAL
Wikipedia releases an annual zim, eg 2019. That is torrented. Then, there are say four updates per year, which arrive Spring Summmer, Autumn Winter. These updates would include the new edits and added material. Something like zsync would be used to update the downloaded torrent to the more recent version.

This would allow an accumulation throughtout the year of people seeding the same torrent, the 2019 one, and at the same time allow people to be up to date without having to re-download the entire wikipedia zim.