EPUB 3 to DAISY 2.02: use subdirectories

We would prefer if only main entrypoints to the book are stored in the top-level directory.

So for the EPUB 3 to DAISY 2.02 script, the ncc.html file should be in the top level directory, but as much as possible of other files should be in subdirectories.

Maybe this could be optional, and specified in an option as the name of the subdirectory that everything should be stored in. For instance, everything except the NCC could be stored in a "daisy202" directory, or maybe someone prefers to just call it "content".

Start at the EPUB 3 to DAISY 2.02 script and trace where ncc.html and the other output files are written. Define how an optional subdirectory name affects the output layout, then verify that ncc.html remains at the top level and other files are placed beneath the selected directory.

Written by the indexing model from the issue text.