Support Epub chapters that are split accross multiple files

[jasmeet0817]

Hi there,

I noticed that this repo doesn't work when Epub chapters are split using split in filename. I've seen Epub archives that only have the first split listed in ncx file and it expects the Epub reader to figure out and merge the other splits. Example:

NCX file ->

This format is recognized and works with Epub readers like "Readera" so I'm assuming this is a normal Epub naming convention.

This PR will merge the splits into the chapter as expected.

Should be a no-op if there is no split.