title: data.frame group_by: latest order date + name
body: <p>I have a <code>data.frame</code> with a bunch of sales information i.e. order.date, buyer.name, buyer.email, etc. I'm using the <code>dplyr</code> package to do a <code>group_by</code> and <code>summarise</code> the information, like this:</p>
  
  <pre><code>sum = 
    df %&gt;%
    select(order.date, buyer.name, buyer.email, item.type, item.name, item.price, quantity, item.total) %&gt;%
    group_by(buyer.email) %&gt;%
    summarise(total.items = sum(quantity), total.amount = sum(item.total)) %&gt;%
    arrange(desc(total.amount))
  </code></pre>
  
  <p>This gives me a list of emails with <code>total.items</code> and <code>total.amount</code> per email. What I also want is the latest <code>order.date</code> and the last <code>buyer.name</code> used for the email. Is there a way to do that with <code>dplyr</code>? If not, what's the most efficient way of going about that?</p>