Strikethrough checked items

client/components/cards/checklists.styl

@@ -141,6 +141,7 @@ textarea.js-add-checklist-item, textarea.js-edit-checklist-item
     &.is-checked
       color: #8c8c8c
       font-style: italic
+      text-decoration: line-through
     & .viewer
       p
         margin-bottom: 2px