title: Split delimited strings in a column and insert as new rows
body: <p>I have a data frame as follow:</p>
  
  <pre><code>+-----+-------+
  |  V1 |  V2   |
  +-----+-------+
  |  1  | a,b,c |
  |  2  | a,c   |
  |  3  | b,d   |
  |  4  | e,f   |
  |  .  | .     |
  +-----+-------+
  </code></pre>
  
  <p>Each of the alphabet is a character separated by comma. I would like to split V2 on each comma and insert the split strings as new rows. For instance, the desired output will be:</p>
  
  <pre><code>+----+----+
  | V1 | V2 |
  +----+----+
  |  1 |  a |
  |  1 |  b |
  |  1 |  c |
  |  2 |  a |
  |  2 |  c |
  |  3 |  b |
  |  3 |  d |
  |  4 |  e |
  |  4 |  f |
  +----+----+
  </code></pre>
  
  <p>I am trying to use <code>strsplit()</code> to spit V2 first, then cast the list into a data frame. It didn't work. Any help will be appreciated.</p>