Tweak output display

_output.yaml

@@ -0,0 +1,10 @@
+bookdown::gitbook:
+  config:
+    toc:
+      collapse: section
+      before: |
+        <li><strong><a href="./">R for Data Science</a></strong></li>
+    edit:
+      link: https://github.com/hadley/r4ds/edit/master/%s
+      text: "Edit"
+    sharing: no