Support multiple input Kindle files

Support multiple input Kindle files

Sample bulk/batch usage:

    @echo off
    setlocal

    SET IN_DIR=%USERPROFILE%\My Documents\My Kindle Content
    SET DEST_DIR=%IN_DIR%_drmfree
    SET KEY_FILE=mykeyfile.k4i

    mkdir "%DEST_DIR%
    k4mobidedrm.py -k %KEY_FILE% "%IN_DIR%" "%DEST_DIR%

    echo done, see %DEST_DIR%
    cd /d "%DEST_DIR%"
    start .

    endlocal