Enforce specific mode at startup

[ghost]

Hey,

First of all thank you for this software, it has really made a difference for me :)

But a feature I would like to see would be to enforce a specific mode (e.g. integrated only) straight from startup. The reasoning behind this is that I'm using Parallels to run some Windows software and it really doesn't play well with dynamic switching, which sometimes causes kernel panics. I've lost quite a lot of work because of this because I forgot to set it to integrated-only or discrete-only after a reboot.

[codykrieger]

Dupe of #150.