title: R convert list of characters and vectors into data.table of characters
body: <p>I've got a list like the following:</p>
  
  <pre><code>    X              Y
  
     'A'           'B'
      c('G','F')     c('E','D') 
  </code></pre>
  
  <p>I need to convert it into a data.table like:</p>
  
  <pre><code> X              Y
  
  'A'           'B'
  'G'           'E'
  'F'           'D' 
  </code></pre>