added _primus completion for primusrun

Added completion for primusrun, which is a script for faster OpenGL offloading for NVIDIA Optimus chips. 

The original author of this script posted on the [zsh-bumblebee](https://aur.archlinux.org/packages/zsh-bumblebee/) AUR package that this completion should be added to the zsh-completions project.

(Also, I used a clean base to branch from :yum:)