refactor (split method)
refactor $file -> $basename
refactor: do not overwrite $name !!
refactor
refactor invert  if - else
refactor
change the style of user's plugin filename into short style!!

old: app/plugin/Logwriter/{appid}_Plugin_Logwriter_File.php
new: app/plugin/Logwriter/File.php
refactor
refactor
refactor
refactor
refactor: DRY!! DRY!!
これは仕様とします。(キリッ