title: Summarising list of columns after group by
body: <p>Is it possible to summarise big number of columns, without writing all their names?</p>
  <p>My example:
  I have a dataframe (dt) with one categorical column and a lot of numeric colunms:</p>
  <pre><code>Cat num1 num2 num3 ... num50
  a   56   59   67   ... 89
  a   46   66   27   ... 59
  b   15   9    75   ... 43
  b   45   29   35   ... 93
  </code></pre>
  <p>I make the following operation:</p>
  <pre><code>dt %&gt;% group_by(Cat) %&gt;% summarize(num1 = sum(num1), num2 = sum(num2), ... num50= sum(num50))
  </code></pre>
  <p>But writing all the 50 column names takes too long time!
  Can I write this summarize expression shorter? I tried this variant, but it doesn't work:</p>
  <pre><code>dt %&gt;% 
    group_by(Cat) %&gt;% 
    summarize(num1:num50 = sum(c(num1:num50)))
  </code></pre>
  <p>Help me, please, how to write it laconically using dplyr of data.table (or other libraries).</p>