title: get pairwise sums of multiple columns in dataframe
body: <p>I have a dataframe that looks like this:</p>
  
  <p><code>x&lt;-data.frame(a=6, b=5:1, c=7, d=10:6)</code></p>
  
  <pre><code>&gt; x
    a b c  d
  1 6 5 7 10
  2 6 4 7  9
  3 6 3 7  8
  4 6 2 7  7
  5 6 1 7  6
  </code></pre>
  
  <p>I am trying to get the sums of columns <code>a</code> &amp; <code>b</code> and <code>c</code>&amp;<code>d</code> in another data frame that should look like:</p>
  
  <pre><code>&gt; new
    ab cd
  1 11 17
  2 10 16
  3  9 15
  4  8 14
  5  7 13
  </code></pre>
  
  <p>I've tried the <code>rowSums()</code> function but it returns the sum of ALL the columns per row, and I tried <code>rowSums(x[c(1,2), c(3,4)])</code> but nothing works. Please help!!</p>