title: How to cumulatively add time
body: <p>I have time interval in minutes.</p>
  
  <pre><code>dfr &lt;- data.frame(interval=c(rep("30",2),rep("60",2)),stringsAsFactors=F)
  
  &gt;&gt; dfr
    interval
  1       30
  2       30
  3       60
  4       60
  </code></pre>
  
  <p>And I have a start time, say 09:00.</p>
  
  <pre><code>time_begin &lt;- "09:00"
  </code></pre>
  
  <p>I would like to start with <code>time_begin</code> and cumulatively add intervals to get the result below:</p>
  
  <pre><code>  interval start   end
  1       30 09:00 09:30
  2       30 09:30 10:00
  3       60 10:00 11:00
  4       60 11:00 12:00
  </code></pre>
  
  <p>What is the best way to do this using dplyr/lubridate?</p>