Show a permalink anchor when hovering over headings in main content (#2251)

* Implement heading permalinks, close #2246

Thanks to jekyll/jekyll for CSS.
Link anchor is visible when the mouse hovers over the title line.

* Build the updated _main.js

.gitignore

@@ -12,4 +12,5 @@ codekit-config.json
 example/_site
 Gemfile.lock
 node_modules
-npm-debug.log*
+npm-debug.log*
+vendor/bundle