[Suggestion] Support multiple proxies

[rraallvv]

I'm having some problems downloading a big file from a server that only supports one connection. I tried Aria2c but it keeps dropping all additional connections, so it'd be really helpful to have something like content-downloader to pass a list of proxies (maybe a text file) so that it keeps trying every proxy in the list until it has established the maximum number of connections, and also to rotate the proxies if some are offline or stop working while the files is being downloaded.