"retries", "skiperror", "delete" features & config options added

    retries: 3                      # retry 3 times if fail to download a gem, optional, def is 1. (no retry)
    delete: false                 # option to control whether delete gems (if remote ones are removed),optional, default is false. 
    skiperror: true              # control whether script skips error, optional, def is true. set it to false, script will stop at error.
please align comments