Add option to enable 'cheats' in downloaded world

[johnmlang]

Feature Request:

Add a toggle option to the World Downloader options to enable 'cheats', also called Server Commands, in the downloaded single player world.

This preference is stored in the level.dat file with the NBT byte tag 'alllowCommands' . Commands are on when the value is 1 and off when the value is 0.

[jrtc27]

Seconded