Putting prefix mapping in the config; FFA spawn

This *SHOULD* be the "final" version of this example, just in time for
the patch's release
- Adding the map prefix mapping to DefaultInstagib.ini, and changing the
code to work in the same way UTGame handles DefaultMapPrefixes. This is
pretty portable; other mods could copy this straight up, treat it as
boilerplate (the same can be said for most of this mod)
- Set MinimumRespawnTime to 0 so that FFA will also have faster respawn
times
Merge branch 'master' of https://github.com/BradyBrenot/chivalry-instagib