Fix description of epub subcommand --filename option

We want to use @filename as basename of generated EPUB filename or
fullpath of generated EPUB filename.
If specify basename, epub subcommand generates EPUB file to @outputdir + @filename.
If specify fullpath, epub subcommand generates EPUB file to @filename.

TODO: Implement this behavior in BitClust::Generator::EPUB class.
Imprement epub subcommand