readded hugo-book theme files.

themes/hugo-book/layouts/_shortcodes/tab.html

@@ -0,0 +1,12 @@
+{{- $group := printf "tabs-%v" .Parent.Ordinal -}}
+{{- $tab := printf "%s-%d" $group .Ordinal -}}
+<input type="radio" class="toggle" name="{{ $group }}" id="{{ $tab }}" {{ if not .Ordinal }}checked="checked"{{ end }} />
+{{- /* remove whitespaces */ -}}
+<label for="{{ $tab }}">
+  {{- .Get 0 -}}
+</label>
+{{- /* remove whitespaces */ -}}
+<div class="book-tabs-content markdown-inner">
+{{ .Inner -}}
+</div>
+{{- /* remove whitespaces */ -}}