Add method for creating links which open in new tabs.

layouts/shortcodes/external-link.html

@@ -0,0 +1,4 @@
+{{   $text := .Get "text" | default (.Get 0)
+}}{{ $url := .Get "url" | default (.Get 1)
+}}
+<a href="{{ $url }}" target="_blank" rel="noopener noreferrer">{{ $text }}</a>