.gitignore: add config.json

Signed-off-by: Young-Ho Cha <ganadist@gmail.com>
fetcher: fix spawn lifecycle

Signed-off-by: Young-Ho Cha <ganadist@gmail.com>
main: improve download management

add DownloadManager
remove unused ogg file when new soundlist file fetched
do not fetch again files that already downloaded
if fetch was failed, retry after 10 minutes

Signed-off-by: Young-Ho Cha <ganadist@gmail.com>