add xml:space="preserve" to text runs

In MS Word 2015 (and possibly others), leading and trailing spaces are
ignored in text runs. This means that calling TextRun with
leading/trailing space would result in a document that didn't include
those spaces. The fix here (per http://officeopenxml.com/WPtext.php)
is to include an extra attribute on the "w:t" element that forces word
to recognize those spaces.

ts/docx/run/run-components/text.ts

@@ -1,9 +1,13 @@
-import { XmlComponent } from "../../xml-components";
+import { XmlAttributeComponent, XmlComponent } from "../../xml-components";
+
+class TextAttributes extends XmlAttributeComponent<{space: "default" | "preserve"}> {
+    protected xmlKeys = {space: "xml:space"};
+}
 
 export class Text extends XmlComponent {
-
     constructor(text: string) {
         super("w:t");
+        this.root.push(new TextAttributes({space: "preserve"}));
         if (text) {
             this.root.push(text);
         }