Add markup renderers

Signed-off-by: Nikolaos Karaolidis <nick@karaolidis.com>

assets/sass/heading.scss

@@ -0,0 +1,10 @@
+.heading {
+  .heading-anchor {
+    opacity: 0;
+    transition: opacity 0.2s ease;
+  }
+}
+
+.heading:hover .heading-anchor {
+  opacity: 1;
+}

assets/sass/main.scss

@@ -6,6 +6,7 @@ $width-max: 60rem;
 @import "fonts";
 @import "common";
 
+@import "heading";
 @import "image";
 
 @import "header";