title: Replacement for diff() for multiple columns
body: <p>diff() calculates the difference between values in a vector at a specified lag.
  Is there an equivalent function that works on two vectors? For example, I have:</p>
  
  <pre><code>v1 = c(1, 2, 3, 4, 5, 3)
  v2 = c(5, 4, 3, 2, 1, 0)
  </code></pre>
  
  <p>I need to calculate the difference between each value of v1 and v2 at lag 1. That would be:</p>
  
  <pre><code>(2 - 5), (3 - 4), (4 - 3)... 
  </code></pre>
  
  <p>This can be achieved using combinations of head()/tails() on the 2 vectors, but I was wondering if there is already a function that can do the same.</p>