Allow MiradorViewer to not immediately render onto the page

__tests__/src/lib/MiradorViewer.test.js

@@ -10,7 +10,7 @@ describe('MiradorViewer', () => {
   beforeAll(() => {
     ReactDOM.render = jest.fn();
     ReactDOM.unmountComponentAtNode = jest.fn();
-    instance = new MiradorViewer({});
+    instance = new MiradorViewer({ id: 'mirador' });
   });
   describe('constructor', () => {
     it('returns viewer store', () => {

src/lib/MiradorViewer.js

@@ -25,7 +25,7 @@ class MiradorViewer {
       || createStore(getReducersFromPlugins(this.plugins), getSagasFromPlugins(this.plugins));
     this.processConfig();
 
-    ReactDOM.render(
+    config.id && ReactDOM.render(
       <Provider store={this.store}>
         <HotApp plugins={this.plugins} />
       </Provider>,
@@ -48,7 +48,7 @@ class MiradorViewer {
    * Cleanup method to unmount Mirador from the dom
    */
   unmount() {
-    ReactDOM.unmountComponentAtNode(document.getElementById(this.config.id));
+    this.config.id && ReactDOM.unmountComponentAtNode(document.getElementById(this.config.id));
   }
 }