title: Saving version comments with paper_trail
body: <p>I have a model (<code>Candidate</code>) that I need to keep track of changes, version diffs, who changed it and any comments they had regarding the change.  I love paper_trail and it is exactly what I need in almost every way, but I'm wondering what the best way to store "change comments" is.  I've been thinking of implementing one of three options:</p>
  
  <ol>
  <li><p>Add a <code>comments</code> column to my <code>versions</code> table.  If I went this way I'm not quite sure of the best way to actually update that information.  Maybe <code>candidate.version.comments = @comments</code> would work?  Is it bad to add custom columns to paper_trail's table?</p></li>
  <li><p>Use paper_trail's metadata to store the comment.  It doesn't seem like this was the intention for the metadata, but maybe it would work?</p></li>
  <li><p>Add another table as <code>belongs_to :candidate</code> that would keep track of the <code>comments</code> and <code>version</code>. This seems gross - would I also need to have this new table belong to the versions table?</p></li>
  </ol>
  
  <p>Any insight would be much appreciated.  I've been pretty focused on finding a solution with paper_trail since I've used it before but I'd also consider other gems or options that you've used in the past.</p>