feat(DefaultRenderer): custom color

**Is your feature request related to a problem? Please describe.**
To avoid having the recreate the renderer just to be ab le to change the color, letting user change those color while instantiating the class would be useful

**Describe the solution you'd like**
Add two color variable that can be set through class instantiation.

**Additional context**
The Idea

```typescript
export class DefaultRenderer implements Renderer {
underMeanColor: string
overMeanColor: string

constructor(underMeanColor = '#0000ff', overMeanColor = '#ff0000') {
this.underMeanColor = underMeanColor
this.overMeanColor = overMeanColor
super()
}

public render( { count, position }: Cluster, stats: ClusterStats, map: google.maps.Map): Marker {
// change color if this cluster has more markers than the mean cluster
const color =
count > Math.max(10, stats.clusters.markers.mean) ? this.overMeanColor : this.underMeanColor;

// ...
}
```