Add demo for custom xml styles

demo/demo13.js

@@ -0,0 +1,26 @@
+// This example shows 3 styles
+const fs = require('fs');
+const docx = require('../build');
+
+const styles = fs.readFileSync('./demo/assets/custom-styles.xml', 'utf-8');
+const doc = new docx.Document({
+  title: 'Title',
+  externalStyles: styles
+});
+
+doc.createParagraph('Cool Heading Text').heading1();
+
+let paragraph = new docx.Paragraph('This is a custom named style from the template "MyFancyStyle"');
+paragraph.style('MyFancyStyle');
+doc.addParagraph(paragraph);
+
+doc.createParagraph('Some normal text')
+
+doc.createParagraph('MyFancyStyle again').style('MyFancyStyle');
+paragraph.style('MyFancyStyle');
+doc.addParagraph(paragraph);
+
+var exporter = new docx.LocalPacker(doc);
+exporter.pack('My Document');
+
+console.log('Document created successfully at project root!');