Add 'align' option to figure shortcode (#256)

Usage: 

{{< figure align=center src="image.jpg" >}}

Parameter values:
  - 'center': align image and captions to center of page
  - anything else neglected

* Appends "#center" to the image's URL in order to utilize existing CSS
rules for centering images.

assets/css/post-single.css

@@ -242,6 +242,10 @@
     margin: auto
 }
 
+.post-content figure.align-center {
+	text-align: center;
+}
+
 .post-content figure > figcaption {
     color: var(--primary);
     font-size: 16px;