Got underline working in editor

Major step, since this is the first inline HTML element which needed
advanced parsing out on the markdown side, since not commonmark
supported.

resources/js/editor/util.js

@@ -13,4 +13,39 @@ export function docToHtml(doc) {
     const renderDoc = document.implementation.createHTMLDocument();
     renderDoc.body.appendChild(fragment);
     return renderDoc.body.innerHTML;
+}
+
+/**
+ * @class KeyedMultiStack
+ * Holds many stacks, seperated via a key, with a simple
+ * interface to pop and push values to the stacks.
+ */
+export class KeyedMultiStack {
+
+    constructor() {
+        this.stack = {};
+    }
+
+    /**
+     * @param {String} key
+     * @return {undefined|*}
+     */
+    pop(key) {
+        if (Array.isArray(this.stack[key])) {
+            return this.stack[key].pop();
+        }
+        return undefined;
+    }
+
+    /**
+     * @param {String} key
+     * @param {*} value
+     */
+    push(key, value) {
+        if (this.stack[key] === undefined) {
+            this.stack[key] = [];
+        }
+
+        this.stack[key].push(value);
+    }
 }