Keep state, better navigation

[TobiX]

This would probably mean a pretty significant core change. Currently, dosage always starts from the latest comic (or one specified by an ID) and works its way backwards from there. This can lead to the following results:

• In a perfect world, dosage fetches each strip until it reaches the very first one and then stops. Additional calls to dosage only fetch new images if new strips were added to the end since the last run
• If dosage was rudely interrupted (network outage, server error, user interrupt), you now have a partial download with a chunk missing in the front. The only way to retrieve these is
  • either run dosage with --all to blindly step over all existing strips in the hope of getting farther this time (wasting time & bandwidth for moving past existing strips)
  • or try to manually find the correct ID to resume from (if possible) and restart dosage with that ID
• Individual strips might fail to download (images, not the HTML), so now you have gaps in your comic archive and the best way forward is (again) running dosage with --all in the hope of fixing those gaps

Most of those cases could be avoided it dosage kept some state and could be told to "finish" fetching a comic from there... This would also open the way to a more "natural" fetching strategy starting from the first strip of a comic. Over the years I encountered some (hand-written) comics were backward-navigation was plainly broken (because it was never tested) or lead to awkward loops...