Add shortcodes for customizing slides, markdown, HTML

layouts/shortcodes/markdown.html

@@ -0,0 +1,5 @@
+<section data-noprocess data-markdown data-separator="^\r?\n---\r?\n$">
+  <script type="text/template">
+    {{ .Inner }}
+  </script>
+</section>

layouts/shortcodes/slide.html

@@ -0,0 +1,9 @@
+{{ $attributes := slice "background-color" "background-image" "background-size" "background-position" "background-repeat" "transition" "background-iframe" "background-interactive" "background-video" "background-video-loop" "background-video-muted" "background-size" }}
+{{ $params := . }}
+<section data-noprocess
+  {{ range $attribute := $attributes -}}
+    {{- with $params.Get $attribute }}data-{{ $attribute | safeHTMLAttr }}="{{ . }}"{{ end -}}
+  {{- end -}}
+  >
+{{ .Inner }}
+</section>