feat: onComplete config option (#4322)

* feat: `onComplete` config option

* tests(e2e): add case for onComplete option

test/e2e/scenarios/on-complete.js

@@ -0,0 +1,22 @@
+const expect = require("expect")
+
+describe("onComplete option", function () {
+  let mainPage
+  beforeEach(function (client, done) {
+    mainPage = client
+      .url("localhost:3200")
+      .page.main()
+
+    client.waitForElementVisible(".opblock-tag-section", 5000)
+    done()
+  })
+
+  it("triggers the page-provided onComplete exactly 1 times", function (client, done) {
+    client.execute(function() {
+      return window.completeCount
+    }, [], (result) => {
+      expect(result.value).toEqual(1)
+      client.end()
+    })
+  })
+})