Allow single pages (#35)

* Let users create single sites

* Create shortcodes for bulma (sub)titles

* Support sidebar in singlepages only

* Fix shortcodes according to the bulma doc

* Use h5 tag for is-5 class

layouts/partials/single/content.html

@@ -0,0 +1,12 @@
+<section class="section is-medium">
+  <div class="container">
+    <div class="columns">
+      <div class="column is-centered-tablet-portrait">
+        <h1 class="title section-title">{{ .Title }}</h1>
+        <h5 class="subtitle is-5 is-muted">{{ .Params.Subtitle }}</h5>
+        <div class="divider"></div>
+      </div>
+    </div>
+    {{ .Content }}
+  </div>
+</section>