Added support for Dark Messiah engine and game. (no bug, r=me).

The development of this feature would not be possible without the support of the following people from the game's community:
Dylan Riggs, Carl Pettengill, Ed Moreland, and Christian.

core/ExtensionSys.cpp

@@ -73,6 +73,8 @@ CLocalExtension::CLocalExtension(const char *filename)
 		"extensions/auto.2.l4d/%s",
 #elif SOURCE_ENGINE == SE_ORANGEBOX
 		"extensions/auto.2.ep2/%s",
+#elif SOURCE_ENGINE == SE_DARKMESSIAH
+		"extensions/auto.2.darkm/%s",
 #else
 		"extensions/auto.2.ep1/%s",
 #endif //SOURCE_ENGINE == SE_LEFT4DEAD