Fix legend and title layout options update

[simonbrunel]

Legend and title layout items must be updated when changing chart options:

var chart = new Chart(ctx, {
  options: {
    legend: {
      position: 'top'
    }
  }
};

chart.options.legend.position = 'left';
chart.update(); // <-- broken layout

[etimberg]

In the interest of completeness, should we add JSDoc comments for the parameters here?

[simonbrunel]

I updated the PR but also changed a bit the configure behavior: only update item properties if there is an option value.

[etimberg]

Looks good to me