Add native Tournament Mode support

This commit adds a Tournament Mode for Arena: Respawn, which can be
activated with ars_tournament_start and deactivated with
ars_tournament_stop. ars_tournament_start also functions as a "reset"
button, should a tournament reset be required.

Tournaments no longer require two separate configuration files that
must be executed manually. For convenience's sake, three configuration
files have been added to cfg/sourcemod for the three states of A:R
play, namely, Public, Pre-Tournament (Deathmatch), and Tournament. At
the completion of a Tournament (when win conditions have been met), the
plugin will fall back to Pre-Tournament mode until further intervention
from an admin or user with console access.

For more information about Tournament Mode, refer to the README file.