Project: svg-chart
Task (challenging): In `common/PieChart.js`, class `PieChart` inherits from `Chart`, `PieChart`:
- append g element (class `datasets`) inside `svg` and make it fill the most area of `svg`
- append g element (class `dataset dataset-0`) inside `.datasets` for the first `datasets` item
- append sectors (path elements, each class `sector sector-{index}`) with the unique color inside `.dataset` for each `dataset.data` item
- all the sectors form a large circle
- the area of each sector is related to its proportion of the data

Update `index.js`, create `PieChart` instances when `config.type` is `pie`.