Cheat Codes (DEMO VERSION)
Enable the console from the game's in-game options.
Hit the tilde key (~) to drop the dev-console.
Type seta thereisacow "1337" and then press ENTER.
Type spdevmap bog_a and then press ENTER.
This will start the demo with cheats enabled.
When playing, drop down the dev-console and enter
the following codes and hit ENTER to activate or toggle them.

Cheat Codes (COMPLETE VERSION)
Same as above but after you load up the spdevmap bog_a exit out and load the level you are currently at, or a new game.

You can not select "resume" and have the cheats function you can however select the map/act you want to start at with the cheats enabled.