title: Group by multiple columns and sum other multiple columns
body: <p>I have a data frame with about 200 columns, out of them I want to group the table by first 10 or so which are factors and sum the rest of the columns.</p>
  
  <p>I have list of all the column names which I want to group by and the list of all the cols which I want to aggregate.</p>
  
  <p>The output format that I am looking for needs to be the same dataframe with same number of cols, just grouped together.</p>
  
  <p>Is there a solution using packages <code>data.table</code>, <code>plyr</code> or any other?</p>