add support for floating drawings

- added need elements and test for them

src/file/drawing/floating/position-offset.ts

@@ -0,0 +1,9 @@
+// http://officeopenxml.com/drwPicFloating-position.php
+import { XmlComponent } from "file/xml-components";
+
+export class PositionOffset extends XmlComponent {
+    constructor(offsetValue: number) {
+        super("wp:posOffset");
+        this.root.push(offsetValue.toString());
+    }
+}