Add reading time to post header, remove purgecss because it was breaking CSS used by injected HTML.

_includes/post.njk

@@ -9,6 +9,7 @@ title: Adrian Victor:Blog
     <article>
         <div id="postHeader">
             <h1>{{ postTitle }}</h1>
+            <p>{{ i18n[langKey].readingTime }}: {{ content | readingTime }}</p>
             <p>{{ authors or "Adrian Victor" }} - <b>{{ date | postDate }}</b>{% if lastModified | postDate !==  date | postDate %} ({{ i18n[langKey].lastEditedIn }} {{ lastModified | postDate }}){% endif %}</p>
             {% if altLanguages.length > 0 %}
             {{ i18n[langKey].availableInOtherLanguages }}: