Restore sub/sup positioning

Author: rcowsill

Diff for: _sass/jekyll-theme-slate.scss

@@ -130,6 +130,14 @@ strong {
   font-weight: bold;
 }
 
+sub {
+  font-variant-position: sub;
+}
+
+sup {
+  font-variant-position: super;
+}
+
 img {
   position: relative;
   margin: 0 auto;