Add support for series navigation links

Signed-off-by: Danila Fedorin <danila.fedorin@gmail.com>

layouts/partials/serieslink.html

@@ -0,0 +1,16 @@
+<div class="{{ .direction }} wrapper">
+  <a href="{{ .link }}">
+    {{ if eq .direction "previous" }}{{ partial "icon.html" "chevrons-left" }}{{ end }}
+    <div class="title-subtitle">
+      {{ title .direction }} in Series
+      {{ $divider := .term.Params.divider }}
+      {{ if $divider }}
+      {{ $rest := after 1 (split .page.Title $divider) }}
+      <div class="title">{{ delimit $rest $divider }}</div>
+      {{ else }}
+      <div class="title">{{ .page.Title }}</div>
+      {{ end }}
+    </div>
+    {{ if eq .direction "next" }}{{ partial "icon.html" "chevrons-right" }}{{ end }}
+  </a>
+</div>