Save novel data in subdirectory if download.use-subdirectory is true

It use part of code as subdirectory name.
Merge branch 'use-subdirectory' of git://github.com/kenhys/narou into kenhys-use-subdirectory
すでにDL済みの小説は use-subdirectory オプションの影響を与えないようにする
Merge branch 'kenhys-use-subdirectory'