Adds the /fog command that allows you to control the fog.
/fog <target> set <color|start|end> <value> - set the selected value
/fog <target> add <start|end> <value> - adds the selected value to the default value
/fog <target> default <color|start|end> - sets the value to default
When changing color, the <value> is <red> <green> <blue>
