Hope to add incremental update function

[Zacharia2]

Incremental update means that when an EPUB file is uploaded to GitHub only this one EPUB book file is converted, and those that have already been converted do not need to be converted. To reduce the time complexity. Instead of converting all the epub files when uploading one book file.

[Jermolene]

Thanks @Zacharia2 that would be a worthwhile improvement, and should be feasible by having the build process write the converted TWPubs back to the repo. Ideally, the build process should also detect TWPubs that were converted with an earlier version of the converter, and reconvert them too.