Downloaded files are written with mode 0600

Hi,

We would like to see that files are written with octal mode 0644 instead of 0600, so that the cached files are readable by other users of our remirror host. The 0600 appears to be a Go default. I believe that this should be changed [here](https://github.com/yobert/remirror/blob/master/main.go#L194), but I'm not sure how (I am very unfamiliar with Go).