feat(transformer): enable recursive processing in custom handler func…

…tions

The VDOMTransformer's `handler` functions did not allow for recursive processing of child nodes so
far, i.e. when a handler function wanted to process only a single MDAST node, but hand the
processing of all MDAST child nodes back to the original transformer, it had no way of doing so,
making each `handler` function a terminal node. With this change, the signature of `handler`
functions changes to `handler(callback, node, parent, handlechild)` where `handlechild(callback,
childnode, mdastparent, hastparent)` is a callback itself that takes the current HTAST-constructing
`callback`, the MDAST `childnode` that should get transformed, the `mdastparent`, i.e. the current
node from the vantage point of the `handler` function, and with `hastparent` a HAST parent node that
the output of the child processing will be attached to.

src/utils/mdast-to-vdom.js

@@ -106,7 +106,23 @@ class VDOMTransformer {
     const handlefn = that.matches(node);
 
     // process the node
-    const result = handlefn(cb, node, parent);
+
+    /**
+     * A function that enables the recursive processing of MDAST child nodes
+     * in handler functions.
+     * @param {function} callback the HAST-constructing callback function
+     * @param {Node} childnode the MDAST child node that should be handled
+     * @param {Node} mdastparent the MDAST parent node, usually the current MDAST node
+     * processed by the handler function
+     * @param {*} hastparent the HAST parent node that the transformed child will be appended to
+     */
+    function handlechild(callback, childnode, mdastparent, hastparent) {
+      if (hastparent && hastparent.children) {
+        hastparent.children.push(VDOMTransformer.handle(callback, childnode, mdastparent, that));
+      }
+    }
+
+    const result = handlefn(cb, node, parent, handlechild);
 
     if (result && typeof result === 'string') {
       return VDOMTransformer.toHTAST(result, cb, node);