Office.js to provide support for retrieving host or domain url from add-in
Suggestion is derived from here:
https://social.msdn.microsoft.com/Forums/office/en-US/5c843b0a-594d-4511-b42f-7d28c79a5cfe/detecting-if-outlook-addin-is-loaded-through-outlookofficecom-vs-outlooklivecom?forum=appsforoffice&prof=required
Office.js to return the parent URL/domain of an Outlook add-in. This is useful for knowing what Outlook web client the user is loading the add-in. Outlook Office365 vs Outlook.com.
Certain browsers don't allow access to the parent domain due to security.
