Convert EPUBs to KEPUBs on download instead of on sync

[wismna]

First of all, thanks for all the work you put on this project, it's really awesome !

Is your feature request related to a problem? Please describe.
Kobo sync converts all epubs to kepubs if kepubify is present. For big libraries, and big files like manga, syncing is already quite long even without this, and with the conversion it takes even longer.

Describe the solution you'd like
I feel it would be simpler and faster to only convert the epubs on a download request, instead of during the sync process.

Additional context
If this feature gathers interest, and if you don't have the time to do it, I could make the PR with the changes.