title: Laravel : How to customize title of email when sending email with Mailable?
body: <p>I write a <code>mailable</code> to send email when user registers,I do it following the document(<a href="https://laravel.com/docs/5.5/mail" rel="noreferrer">https://laravel.com/docs/5.5/mail</a>):</p>
  
  <p>first,generating a mailable:</p>
  
  <pre><code>php artisan make:mail UserRegistered
  </code></pre>
  
  <p>Ok,thus there is a <code>UserRegistered.php</code> file in the <code>app/Mail</code> directory,and I write the <code>build()</code> method like this:</p>
  
  <pre><code>public function build()
  {
      return $this-&gt;view('emails.activate-user')
                  -&gt;with([
                      'name' =&gt; $this-&gt;user-&gt;name,
                      'url' =&gt; route('activateUser',['token'=&gt;$this-&gt;user-&gt;confirmation_token])
                  ]);
  }
  </code></pre>
  
  <p>The email can be sent successfully,the title of the email is <code>User Registered</code>,I want to customize the title ,how to do it?   </p>