Heatmap high-level api

Add high level api (mini DSL) for heatmap customization (like in candlestick) for simplified color/scale adjustment, because now you need to write
```
fillColor(Stat.count) {
scale = continuous()
}
```

Start by examining the existing candlestick high-level API and the current heatmap customization API. The work is done when heatmap color and scale adjustments can use a simplified mini DSL analogous to candlestick customization.

Written by the indexing model from the issue text.