Grid improvements

[straker]

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

• 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]

• 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];