Add manual page + bash-completion profile

I've created a manpage and a bash-completion profile (latter is based off of optirun) for primusrun; please consider including them, for the convenience of downstream package maintainers who may wish to install these as well. Thanks!

(About the man page, I'm unsure of whether to include an AUTHORS/COPYRIGHT section, and I only left it in because Debian's dh-make templates include it, but feel free to remove it if you want.)