add keepLines and keepNext support for paragraph formatting

This commit adds it to the three places where paragraph formatting can
be applied right now (directly on the paragraph, paragraph styles, and
bullets/numbering styles). I'm separately adding an entry to the wiki
regarding what these methods do (widow/orphan control)

ts/docx/paragraph/index.ts

@@ -8,6 +8,7 @@ import { XmlComponent } from "../xml-components";
 import { Alignment } from "./alignment";
 import { ThematicBreak } from "./border";
 import { Indent } from "./indent";
+import { KeepLines, KeepNext } from "./keep";
 import { PageBreak } from "./page-break";
 import { ParagraphProperties } from "./properties";
 import { ISpacingProperties, Spacing } from "./spacing";
@@ -140,4 +141,14 @@ export class Paragraph extends XmlComponent {
         this.properties.push(new Spacing(params));
         return this;
     };
+
+    public keepNext(): Paragraph {
+        this.properties.push(new KeepNext());
+        return this;
+    }
+
+    public keepLines(): Paragraph {
+        this.properties.push(new KeepLines());
+        return this;
+    }
 }