import: add support to import any xml tab element to XmlComponent (#8)

- make method to import/convert to XmlComponent
 - expose method to add childElement to Header/Footer
 - add tests

src/file/header-wrapper.ts

@@ -3,6 +3,7 @@ import { IMediaData, Media } from "./media";
 import { Paragraph } from "./paragraph";
 import { Relationships } from "./relationships";
 import { Table } from "./table";
+import { XmlComponent } from ".";
 
 export class HeaderWrapper {
     private readonly header: Header;
@@ -35,6 +36,10 @@ export class HeaderWrapper {
         this.header.addDrawing(imageData);
     }
 
+    public addChildElement(childElement: XmlComponent | string) {
+        this.header.addChildElement(childElement);
+    }
+
     public createImage(image: string): void {
         const mediaData = this.media.addMedia(image, this.relationships.RelationshipCount);
         this.relationships.createRelationship(