function _markTags(cm, node) {
        node.children.forEach(function (childNode) {
            if (childNode.isElement()) {
                _markTags(cm, childNode);
            }
        });
        var mark = cm.markText(node.startPos, node.endPos);
        mark.tagID = node.tagID;
    }
Recursively walks the SimpleDOM starting at node and marking
all tags in the CodeMirror instance. The more useful interface
is the _markTextFromDOM function which clears existing marks
before calling this function to create new ones.

@param {CodeMirror} cm CodeMirror instance in which to mark tags
@param {Object} node SimpleDOM node to use as the root for marking