add s and u options to strikethrough or underline SVG text

Author: archmoj

src/lib/svg_text_utils.js

@@ -330,6 +330,8 @@ var TAG_STYLES = {
     // baseline below
     sup: 'font-size:70%',
     sub: 'font-size:70%',
+    s: 'text-decoration: line-through',
+    u: 'text-decoration: underline',
     b: 'font-weight:bold',
     i: 'font-style:italic',
     a: 'cursor:pointer',