[feature request] improve downloading by continue downloading partial downloaded files

[moham96]

One feature I miss from packer Is the ability to continue downloading the partial files.
This has two advantages:
1-If the process is interrupted for some reason, then running yaourt again would continue the previous download so it won't redownload from the beginning (this is useful in big files)
2- Sometimes I already have the files that the PKGBUILD is grabbing from the web so all I have to do is move the files that I have to the package temp folder and packer would check the files for using the hashes in the PKGBUILD and use them(also useful for big files)
Currently, yaourt always deletes old files and download them from the beginning.