title: Aggregate all columns of data.table, without having to reference them by name
body: <p>I'd like to do the equivalent of the following, but with data.table's "by":</p>
  
  <pre><code>dt &lt;- data.table(V1=rnorm(100), V2=rnorm(100), V3=rnorm(100), ...
                   group=rbinom(100,2,.5))
  dt.agg &lt;- aggregate(dt, by=list(dt$group), FUN=mean)
  </code></pre>
  
  <p>I know that I could do this:</p>
  
  <pre><code>dt.agg &lt;- dt[, list(V1=mean(V1), V2=mean(V2), V3=mean(V3)), by=group]
  </code></pre>
  
  <p>But for the case I'm considering I have 100 or so columns V1-V100 (and I always want to aggregate all of them by a single factor, as in aggregate above) so the data.table solution I've got above isn't feasible.</p>