Draft: Rankmirrors systemd service

This merge requests adds a systemd unit and timer for rankmirror. It fetches a list of mirrors from the official mirror list and emits the best n many mirrors to /etc/pacman.d/mirrorlist.

It addresses #136.

You can either enable the rankmirror.service directly, which will cause a re-ranking of mirrors on every boot, or enable the rankmirror.timer, which will re-rank mirrors weekly.

Unfortunately, I could only get this to work by having a script, genmirrorlist that does the actual work, with the systemd unit just calling that and redirecting to /etc/pacman.d/mirrorlist. This is unfortunate, but I think the least bad option. Someone with better systemd fu than I may be able to get the piping + quoting + whatever working inside the system unit file directly.