Make downloading archivefiles easier by using a download manager

[kan-fu]

As suggested by Martien.

A typical download manager like uget and aria2c only needs a download link (or links separated by new line) to download file(s). The plan is to add a helper method to return the download link(s) of the archive files, so users can pipeline them into the download manager. Some documentation updates are also needed to demonstrate how to do that.