Add footnote support to API

demo/demo16.js

@@ -0,0 +1,14 @@
+const docx = require('../build');
+
+var doc = new docx.Document();
+
+var paragraph = new docx.Paragraph("Hello World").referenceFootnote(1);
+
+doc.addParagraph(paragraph);
+
+doc.createFootnote(new docx.Paragraph("Test"));
+
+var exporter = new docx.LocalPacker(doc);
+exporter.pack('My Document');
+
+console.log('Document created successfully at project root!');