Grid improvements

[straker]

I'd like to add the ability to do a few more things with grid:

• [x] set individual row/column alignment. Right now that requires setting the children align/justifySelf property, but would be better as an option to the grid.
• [ ] set individual row/column width/height. Right now the width/height is determined by each cell, but being able to make a row/column wider than that would be great.
• [ ] set grid width/height. That way I can have the grid take up the full screen.
• [ ] padding

[straker]

• [x] Would also like to be able to dynamically update the children of the grid. Something like

grid = Grid({
  children: [childA, childB, childC]
});
// ...
grid.children = [child1, child2, child3];