Add support for Microsoft national clouds via Graph API base URL (#590)

* adding support for Microsoft National Clouds

* Update usage.md

docs/source/usage.md

@@ -208,6 +208,8 @@ The full set of configuration options are:
   - `mailbox` - str: The mailbox name. This defaults to the
       current user if using the UsernamePassword auth method, but
       could be a shared mailbox if the user has access to the mailbox
+  - `graph_url` - str: Microsoft Graph URL.  Allows for use of National Clouds (ex Azure Gov)
+      (Default: https://graph.microsoft.com)
   - `token_file` - str: Path to save the token file
       (Default: `.token`)
   - `allow_unencrypted_storage` - bool: Allows the Azure Identity