You can set Link Whisper to make links to external sites open in a new tab from the Settings. This setting applies to suggested external links and Auto Links, and not to manually created external links.
To enable the setting, open the Settings page. Inside the “General Settings” tab, near the top, is the “Open Links to External Sites in a New Tab” option. Activate the option and save the settings to have all links to external sites open in new tabs.