Add some missing HTML5 elements

Source: https://developer.mozilla.org/en-US/docs/Web/HTML

New, non experimental, non deprecated elements:
- <bdi>
- <data>
- <dialog>
- <picture>
- <rp>
- <rt>
- <ruby>
- <search>
- <slot>
- <template>
- <u>

Close #654

lib/arbre/html/html5_elements.rb

@@ -12,6 +12,7 @@ module HTML
       :audio,
       :b,
       :base,
+      :bdi,
       :bdo,
       :blockquote,
       :body,
@@ -24,11 +25,13 @@ module HTML
       :col,
       :colgroup,
       :command,
+      :data,
       :datalist,
       :dd,
       :del,
       :details,
       :dfn,
+      :dialog,
       :div,
       :dl,
       :dt,
@@ -76,14 +79,20 @@ module HTML
       :option,
       :output,
       :param,
+      :picture,
       :pre,
       :progress,
       :q,
+      :rp,
+      :rt,
+      :ruby,
       :s,
       :samp,
       :script,
+      :search,
       :section,
       :select,
+      :slot,
       :small,
       :source,
       :span,
@@ -96,6 +105,7 @@ module HTML
       :table,
       :tbody,
       :td,
+      :template,
       :textarea,
       :tfoot,
       :th,
@@ -104,6 +114,7 @@ module HTML
       :title,
       :tr,
       :track,
+      :u,
       :ul,
       :var,
       :video,