Add command to save replay midrace

Use command "replaymarker" to mark the current race; at the end, a demo will automatically be saved, and the in-game time you used the command will be added to the demo title. Intended use: when something crazy happens to you, hit your replaymarker bind. You can now rest easy knowing that you won't forget to save the replay, and you can go back to it later and quickly find the moment you were looking for.

I get the feeling that this isn't really the sort of thing that belongs in the base game, since Krew has expressed some desire to implement proper seekable replay markers at some point. Compared to that, this is kind of a hack, but I think it'll be useful to a lot of people regardless.

I also have no idea what branch this should go to or what style guidelines I should be conforming to, and this is the first C I've ever written. :XD: