Add _.if(condition, value_if_true, value_if_false)

Added _.if(condition, value_if_true, value_if_false). Useful for stuff like "_.if(x<0, -50, 20) * 200"
Real-world use case: "tileToReturn.yFromTile = function(y) {return _.if(typeof y === 'number', y, tileToReturn.tileY) * tileHeight;};"

Mixin version:
if(!_.if) {
	_.mixin({"if": function(Bool, This, That) {
		if(Bool) {return This;}
		else {return That;}
	}});
}

The test for this should look something like:
  test("if", function() {
    ok(_.if(true, 1, 2) != _.if(false,1,2), "True and false should return different args.");
  });

But I can't find anywhere where it looks like it should go.

I feel a good if() function is a proper tool in any functional programmer's belt. However, since I am *quite* new to Javascript in general, I perhaps have just not figured out how to make the if(){}else{} format work inline. Any feedback?