Add a Gitbook plugin to allow correct loading of doc page hash URLs.

Per #345, navigating to a docs page URL that includes a hash anchor
causes the page to snap back up to the top.  This is due to Gitbook
showing the sidebar.  The "anker-enable" plugin hacks around that by
setting a short timeout on docs page load, and then changing the page
location back to the hash URL.  This appears to work around the issue.

Author: markerikson

book.json

@@ -3,7 +3,7 @@
   "structure": {
     "summary": "docs/README.md"
   },
-  "plugins": ["edit-link", "prism", "-highlight", "github"],
+  "plugins": ["edit-link", "prism", "-highlight", "github", "anker-enable"],
   "pluginsConfig": {
     "edit-link": {
       "base": "https://github.com/reactjs/redux/tree/master",