Add toc_sticky to stick table of contents

"Stick" table of contents to top of the page by adding:

```
toc: true
toc_sticky: true
```

to a post or page's YAML Front Matter.
Note: only supported with `single` layout.

_layouts/single.html

@@ -35,7 +35,7 @@ layout: default
 
       <section class="page__content" itemprop="text">
         {% if page.toc %}
-          <aside class="sidebar__right">
+          <aside class="sidebar__right {% if page.toc_sticky %}sticky{% endif %}">
             <nav class="toc">
               <header><h4 class="nav__title"><i class="fas fa-{{ page.toc_icon | default: 'file-alt' }}"></i> {{ page.toc_label | default: site.data.ui-text[site.locale].toc_label }}</h4></header>
               {% include toc.html sanitize=true html=content h_min=2 h_max=3 class="toc__menu" %}