Add textOverflow and whiteSpace properties to CSSObject

[xAxis.labels.style](http://api.highcharts.com/highcharts/xAxis.labels.style) describes these properties:

> Use `whiteSpace: 'nowrap'` to prevent wrapping of category labels
>  Use `textOverflow: 'none'` to prevent ellipsis (dots).

types/highcharts/index.d.ts

@@ -1994,6 +1994,8 @@ declare namespace Highcharts {
         position?: string;
         top?: string;
         textOutline?: string;
+        textOverflow?: string;
+        whiteSpace?: string;
     }
 
     interface CreditsOptions {