pie/donut support for negative values

C3 version: `0.7.2`
D3 version: `^5.8.0`
Browser: `Chrome 75`
OS: `Fedora 30`

```javascript
var chart = c3.generate({
data: {
columns: [
['data1', -30],
['data2', 120],
],
type : 'pie',
onclick: function (d, i) { console.log("onclick", d, i); },
onmouseover: function (d, i) { console.log("onmouseover", d, i); },
onmouseout: function (d, i) { console.log("onmouseout", d, i); }
}
});
```

c3 renders:

expected effect, something like: