Adds sup and sub to titles and subtitles (#1343)

* Adds sup and sub to titles and subtitles

* Changes sub and sup to 75% and makes them variables

* Merge .title and .subtitle variables

Merged the .title and .subtitle sub and sup variables into just one $title-sub-size and $title-sup-size

sass/elements/title.sass

@@ -3,6 +3,8 @@ $title-size: $size-3 !default
 $title-weight: $weight-semibold !default
 $title-strong-color: inherit !default
 $title-strong-weight: inherit !default
+$title-sub-size: 75% !default
+$title-sup-size: 75% !default
 
 $subtitle-color: $grey-dark !default
 $subtitle-size: $size-5 !default
@@ -17,6 +19,10 @@ $subtitle-strong-weight: $weight-semibold !default
   em,
   span
     font-weight: inherit
+  sub
+    font-size: $title-sub-size
+  sup
+    font-size: $title-sup-size
   .tag
     vertical-align: middle