title: Rowsums conditional on column name
body: <p>My data.frage looks like this: </p>
  
  <pre><code>   VAR1 VAR2 AUS1 AUS2 AUS3 AUS4 ... AUS56 VAR3 VAR4
     A    D    23   234  34   856  ... 99    0    FCK
     B    D    55   76   55   36   ... 6456  0    XYC
  </code></pre>
  
  <p>I'd like R to add a new variable <code>AUS</code> which shows the rowsums of the variables <code>AUS1</code> to <code>AUS56</code>, preferably with dplyr. <code>AUS1</code> to <code>AUS56</code> can then be deleted.</p>