added a downloadRedirect option to send only a link to the download-file

A simple hack for being able to download long files without having to increase the php-memory-limit. Set

```
downloadRedirect
```

to a regular expression (like "/^.srv.http./" for ArchLinux) that will be used to replace the file-path of the download-link.

If downloadRedirect is false, it behaves as before and tries to send the file directly.

I'm sure the serverRoot or fileRoot could be used for this, too...