title: data.table substring on column
body: <p>Given this data.table</p>
  
  <pre><code>DT = data.table(item=c("item1 - description one", "item2 - description two", "item3 - description three"), sales=1:3)
  
  DT
                          item sales
  1:   item1 - description one     1
  2:   item2 - description two     2
  3: item3 - description three     3
  </code></pre>
  
  <p>How can I easily get output that looks like</p>
  
  <pre><code>      code sales
  1:   item1     1
  2:   item2     2
  3:   item3     3
  </code></pre>
  
  <p>Probably very easy but thanks in advance.</p>