Implement use cases of launcher IPC

[rzaba0]

List of use cases is here: launcher-IPC-use-cases.pdf. We decided to implement cases 2, 7, 3, and maybe 1.
Also refer to #149 for more information on launcher IPC.

Implementing the actual use cases will show whether we can stick to a simple implementation of messages, or we'll need more. Here are some ideas:

• Consider formatting the messages as JSON
• Add ability to send multiple commands/cvar settings at once - this could be very convenient when saving settings in launcher, unless there's a better way to do it (e.g. with /loadcon command)