allow Underline to be called with type and color; fix default

ts/docx/run/formatting.ts

@@ -1,4 +1,5 @@
 import { Attributes, XmlComponent } from "../xml-components";
+export { Underline } from './underline';
 
 export class Bold extends XmlComponent {
 

ts/docx/run/underline.ts

@@ -13,8 +13,8 @@ abstract class BaseUnderline extends XmlComponent {
 
 export class Underline extends BaseUnderline {
 
-    constructor() {
-        super("");
+    constructor(underlineType: string = "single", color?: string) {
+        super(underlineType, color);
     }
 }