add ability to change size while maintaining the center


More often than not when I wanted to change the size of many things, I wanted the center to be maintained. This is a very tiny change but likely commonly used. 

````javascript
var rect = draw.rect(50,50).center(50,50);
...
rect.sizeCentered(40,40); // maintains the center
````

The syntax might need to be changed (perhaps a flag to `size` instead?).