Build a dropdown menu using a click event and .toggle() animation method.
Birdman wants to add a dropdown menu to their home page. Here's what it looks like. Click on the menu icon 
Look at index.html. In the <div class="dropdown"> section, there are two elements: an <img> element and a <ul class="dropdown-menu"> element. When the <img> is clicked, we want the <ul class="dropdown-menu"> element to appear.
In app.js, attach an event handler to the <img> element so that it can respond to a click event.
Inside the click event handler, toggle the <ul class="dropdown-menu"> element.
In index.html, add text for more list items in the dropdown menu. We used for Tools, Blog, Careers, About, and Contact. Feel free to use your own.