Allow cloning PKGBUILDs with git+ssh #749

I've been having some trouble with timeouts while pulling PKGBUILD updates lately. Switching remotes to git+ssh instead of git+https seems to help with this, but pacaur doesn't allow customizing this without editing the script.

I think it would be useful to add a config file option to allow switching this out. The AUR site itself shows git+ssh URLs for cloning.

Additionally, it looks like DownloadPkgs doesn't use $aururl, which is probably an oversight.