title: consecutively subtracting columns in data.table
body: <p>Suppose I have the following data.table:</p>
  
  <pre><code> player_id prestige_score_0 prestige_score_1 prestige_score_2 prestige_score_3 prestige_score_4
     1:    100284     0.0001774623     2.519792e-03     5.870781e-03     7.430179e-03     7.937716e-03
     2:    103819     0.0001774623     1.426482e-03     3.904329e-03     5.526974e-03     6.373850e-03
     3:    100656     0.0001774623     2.142518e-03     4.221423e-03     5.822705e-03     6.533448e-03
     4:    104745     0.0001774623     1.084913e-03     3.061197e-03     4.383649e-03     5.091851e-03
     5:    104925     0.0001774623     1.488457e-03     2.926728e-03     4.360301e-03     5.068171e-03
  </code></pre>
  
  <p>And I want to find the difference between values in each column starting from column <code>prestige_score_0</code> </p>
  
  <p>In one step it should look like this: <code>df[,prestige_score_0] - df[,prestige_score_1]</code></p>
  
  <p>How can I do it in data.table(and save this differences as data.table and keep player_id as well)?</p>