feat(plugins): add Google Analytics plugin (#66)

src/plugins/ga.js

@@ -0,0 +1,42 @@
+// From https://github.com/egoist/vue-ga/blob/master/src/index.js
+
+function appendScript () {
+  const script = document.createElement('script')
+  script.async = true
+  script.src = 'https://www.google-analytics.com/analytics.js'
+  document.body.appendChild(script)
+}
+
+function init (id) {
+  if (!window.ga) {
+    appendScript()
+    window.ga = window.ga || function () {
+      (window.ga.q = window.ga.q || []).push(arguments)
+    }
+    window.ga.l = Number(new Date())
+    window.ga('create', id, 'auto')
+  }
+}
+
+function collect () {
+  init(window.$docsify.ga)
+  window.ga('set', 'page', location.href)
+  window.ga('send', 'pageview')
+}
+
+const install = function () {
+  if (!window.Docsify || !window.Docsify.installed) {
+    console.error('[Docsify] Please load docsify.js first.')
+    return
+  }
+
+  if (!window.$docsify.ga) {
+    console.error('[Docsify] ga is required.')
+    return
+  }
+
+  collect()
+  window.$docsify.plugins = [].concat(window.$docsify.plugins, collect)
+}
+
+export default install()