Reduce duplicated code and automate more things

Also change styling for article captions (author/when published)

_includes/article-heading.html

@@ -0,0 +1,9 @@
+<div class="parallax-section article-title-section" data-jarallax data-speed="{{ site.parallax-speed }}" style="background-image: url('/images/{{ page.url | relative_url | remove: '.html' | replace: '/', '-' | remove: '-articles-' }}.jpg');">
+    <h2 class="mdc-typography--headline3">{{ page.title }}</h2>
+</div>
+<figcaption class="mdc-typography--caption-text">{{ page.image-caption }} &bull; &copy; {{ page.copyright }}</figcaption>
+
+<h3 class="mdc-typography--subtitle1">
+    <time class="timeago" datetime="{{ page.date | date: '%Y-%m-%d' }}T{{ page.date | date: '%H:%M:%S' }}">{{ page.date | date: '%A %e %B %Y' }}</time>
+     &bull; by {{ page.authors }}
+</h3>