Support passing launcher information to mods

[keton]

This change together with https://github.com/praydog/uevr-frontend/pull/9 will allow one click 'launch to VR' from within Rai Pal. This also introduces proper division of responsibility where Rai Pal knows what to launch and UEVRInjector knows how to launch it.

On top of the code change UEVR rai-pal-manifest.json needs to look like this:

{
  "version": "1.03",
  "runnable": {
    "path": "UEVRInjector.exe",
    "args": [
      "--attach={{ExecutablePath}}", "--delay=30", "--launch={{StartCommand}}", "--launch_args={{StartCommandArgs}}"
    ]
  },
  "engine": "Unreal",
  "unityBackend": null
}

PS. this is the first thing I wrote in Rust ever. Feel free to treat it as a suggestion and provide better version instead.

Closes #111