Project: svg-chart
Task (moderate): In `common/Chart.js`, when `options.grids` is true or not defined:
- append g element (class `grids`) in `svg`
- append grid-x lines (each class `grid grid-x grid-x-{index}`, color gray) and grid-y lines (each class `grid grid-y grid-y-{index}`, color gray) in `.grids`
- Ensure that `.grids` is placed under `.datasets`, and that its size matches `.datasets`
- Each grid-x line is horizontally aligned with axis-x labels
- Each grid-y line is vertically aligned with axis-y labels