New Extractor Idea: Generate e-book files (epub/mobi) from archived article text

## Type

- [ ] General question or discussion
- [x] Propose a brand new feature
- [ ] Request modification of existing behavior or design

## Describe the ideal specific solution you'd want, and whether it fits into any broader scope of changes

I think it would be cool to have an integrated [ebook-convert](https://manual.calibre-ebook.com/generated/en/ebook-convert.html) tool from Calibre (or something like it - this one is just the best one I know for that purpose) you can access from Archivebox web UI/command line. For a web UI I'm thinking something like: you tick the needed boxes, and where you now have "pull", "reset" and other options you would also have a "convert to e-book" option. Wanting to browse your articles comfortably on a dedicated e-ink/reader device seems like a common thing - would be neat to have an option to convert articles in bulk straight from your archive. And since Archivebox already archives pdf's, this option shouldn't seem out of place.

## How badly do you want this new feature?

- [ ] It's an urgent deal-breaker, I can't live without it
- [ ] It's important to add it in the near-mid term future
- [x] It would be nice to have eventually

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- [x] I like ArchiveBox so far / would recommend it to a friend

Start by reviewing the web UI and command-line entry points for archive actions, then examine how Calibre's ebook-convert tool handles archived article text. Define the scope for bulk conversion and the supported epub/mobi outputs; done means the proposed conversion is available from the relevant interfaces and works across an archive.

Written by the indexing model from the issue text.