add a history example with tests

docs/src/demos/Examples/History/index.spec.js

@@ -0,0 +1,36 @@
+context('history', () => {
+  beforeEach(() => {
+    cy.visit('/examples/history')
+  })
+
+  describe('undo', () => {
+    it('should not have a mistake', () => {
+      cy.get('.ProseMirror').window().then(window => {
+        const { editor } = window
+        const html = editor.html()
+
+        cy.get('.ProseMirror h2:first').should('not.contain', 'Mistake')
+      })
+    })
+
+    it('should have a mistake', () => {
+      cy.get('.ProseMirror').window().then(window => {
+        const { editor } = window
+        const html = editor.html()
+
+        editor.insertText('Mistake')
+        cy.get('.ProseMirror h2:first').should('contain', 'Mistake')
+      })
+    })
+
+    it('the mistake should be removed again', () => {
+      cy.get('.ProseMirror').window().then(window => {
+        const { editor } = window
+        const html = editor.html()
+
+        editor.undo()
+        cy.get('.ProseMirror h2:first').should('not.contain', 'Mistake')
+      })
+    })
+  })
+})