Add config file option for server-autostart feature

[DuFF14]

From discussion on https://github.com/OSVR/OSVR-Unity/pull/176, here is a summary of points made by @JeroMiya and @demonixis

• Prioritize VR plugins and conditionally auto-start OSVR when/if-and-only-if OSVR is actually being used, instead of enabling/disabling auto-start on a per-app basis. We should look into ways to make prioritization and lazy initialization easier for Unity developers.
• Users should have control over toggling auto-start on/off, across all apps. This makes application behavior more consistent for the user.

We could do it a couple of ways:

• A server config option. Plus add an -autostart argument to the server to let it know that it's being launched with the auto-start API, and should make note of the setting. This is my preference.
• A second environment variable to toggle auto-start. Kind of kludgy but gets the job done.