Add a config.js file to the Chrome extension

This allows the Chrome extension to be configured with hypothesisConfig.
Also moved this file and the destroy.js file into an extension
subdirectory to avoid confusion with the main app files.

h/static/extension/config.js

+function hypothesisConfig() {
+  return {
+    firstRun: location.pathname.indexOf('/welcome') === 0,
+    showHighlights: true
+  };
+}