New chart type request: candlestick charts

I think it would be useful to have a type of timeseries chart that renders as a candlestick chart. The config could look something like this

```
var chart = c3.generate({
data: {
x: 'x',
columns: [
['x', '2013-01-01', '2013-01-02', ...],
['data1', {high: 10, low: 4, open: 9, close: 8}, ...]
]
},
axis: {
x : {
type: 'timeseries'
}
}
}
```

Start by tracing how the existing timeseries chart configuration and rendering handle data columns. Use the proposed x-axis and OHLC object shape as the starting point, then determine the rendering and interaction requirements for candlesticks. Done means a timeseries chart can accept the shown data and display candlestick values correctly.

Written by the indexing model from the issue text.