Added prime-run as an alternative to the variables

[MilesBHuff]

Resolves issue #243.

[jacobgkau]

Per #243, system76-power and nvidia-prime conflict with each other, so we shouldn't recommend the prime-run command specifically; stating that a script can be used instead of manually using the environment variables on every launch would be fine.

[MilesBHuff]

Interesting. On Arch/Manjaro, there is no such conflict, so I actually have prime-run available. Maybe the AUR package needs to be updated to indicate that nvidia-prime and system76-power should not be co-installed?

Here's an idea: what if system76-power bundled prime-run? (Discussion continuing in #243.)