feat(gatsby-plugin-google-analytics): add head option (#3995)

Add head option to configure whether scrip tag should be put in the
body (by default, same as previously) or inside of the head.
closes #2894

packages/gatsby-plugin-google-analytics/README.md

@@ -15,6 +15,8 @@ plugins: [
  resolve: `gatsby-plugin-google-analytics`,
  options: {
  trackingId: "YOUR_GOOGLE_ANALYTICS_TRACKING_ID",
+ // Puts tracking script in the head instead of the body
+ head: false,
  // Setting this parameter is optional
  anonymize: true,
  },

packages/gatsby-plugin-google-analytics/src/gatsby-ssr.js

@@ -1,8 +1,14 @@
 import React from "react"
 
-exports.onRenderBody = ({ setPostBodyComponents }, pluginOptions) => {
+exports.onRenderBody = (
+ { setHeadComponents, setPostBodyComponents },
+ pluginOptions
+) => {
  if (process.env.NODE_ENV === `production`) {
- return setPostBodyComponents([
+ const setComponents = pluginOptions.head
+ ? setHeadComponents
+ : setPostBodyComponents
+ return setComponents([
  <script
  key={`gatsby-plugin-google-analytics`}
  dangerouslySetInnerHTML={{