[FR] Separate epubs per article / Aside: KOreader intregration

**Is your feature request related to a problem?**

I lose my progress reading long articles when new articles get added, and need to search for my place again.

**Describe the solution you'd like**

Omnivore-epub could make different epubs for each article in the list, so progress could be kept for long articles. This could be a toggle, so the current functionality could remain for those that prefer one epub with everything in it.

**Describe alternatives you've considered**

Alternatively, the user could specify a certain article length that warrants a standalone epub vs. being grouped in with other articles.

We could also use omnivore's labeling system to determine whether articles should be in their own epub or in the main epub.

I'm not sure how to keep progress if articles are all in one file unless we integrate with peoples' epub readers. KOreader has a feature called [Progress sync](https://github.com/koreader/koreader/wiki/Progress-sync) which could be used to save progress and place it back at the same place in the article they were reading. This would come with the added benefit forwarding progress back to omnivore, similar to https://github.com/agrmohit/omnivore-epub/issues/2. This change seems much more complicated, however.

Aside: There's also a "Move to archive" feature in KOreader, which could be used with the proposed "separate epubs" feature to allow the user to archive omnivore articles from KOreader.