title: add a value to localstorage, instead of replacing it
body: <p>i want the value from <code>newUsername</code> to be added to the <code>localStorage</code> <strong>setUsernamesArray</strong>, whereas in my code, it replaces the entire value.</p>
  <h3>Code</h3>
  <pre class="lang-js prettyprint-override"><code>$('#signUpButton').click(function() {
    var newUsername = $('#usernameSignInBox').val();
    signedUpUsernames.push(newUsername);
    localStorage.setItem('setUsernamesArray', signedUpUsernames);
  });
  </code></pre>