Impossible to expand

The `processNode` function from the `GeneratingProvider` interface requires a specific implementation of the `HtmlGenerator.HtmlGeneratingVisitor` class rather than an abstraction while `HtmlGenerator.HtmlGeneratingVisitor` implements the `RecursiveVisitor` interface.
```kotlin
interface GeneratingProvider {
fun processNode(visitor: HtmlGenerator.HtmlGeneratingVisitor, text: String, node: ASTNode)
}
```
Please replace `HtmlGenerator.HtmlGeneratingVisitor` with `RecursiveVisitor `in the parameters of the `GeneratingProvider` interface so you can expand or create your own generator! Also, make the class `HtmlGenerator` open so that it can be extended.