Put metadata tags into partial

And update example config.toml (relate to #40)

layouts/partials/structured-data.html

@@ -0,0 +1,6 @@
+{{/* We use some Hugo built-in templates, you can find their source here: */}}
+{{/* https://github.com/gohugoio/hugo/tree/master/tpl/tplimpl/embedded/templates */}}
+
+{{- template "_internal/schema.html" . }}
+{{- template "_internal/opengraph.html" . }}
+{{- template "_internal/twitter_cards.html" . }}