title: Rownames as column in list of dataframes
body: <p>I'd like to create a column <em>year</em> for each dataframe in a list based on its rownames. This question <a href="https://stackoverflow.com/questions/18403199/convert-row-names-in-multiple-data-frames-to-column-in-data-frame">has been asked before</a> on SO but unfortunately the answer doesn't help. So, <em>is</em> there a way to do it?</p>
  
  <pre><code>mylist &lt;- list(structure(list(a = 1:10), .Names = "a", row.names = 1991:2000, class = "data.frame"), 
              structure(list(a = 1:10), .Names = "a", row.names = 1992:2001, class = "data.frame"))
  </code></pre>
  
  <p>Expected outcome:</p>
  
  <pre><code>[[1]]
        a year
  1991  1 1991
  1992  2 1992
  1993  3 1993
  1994  4 1994
  1995  5 1995
  1996  6 1996
  1997  7 1997
  1998  8 1998
  1999  9 1999
  2000 10 2000
  
  [[2]]
        a year
  1992  1 1992
  1993  2 1993
  1994  3 1994
  1995  4 1995
  1996  5 1996
  1997  6 1997
  1998  7 1998
  1999  8 1999
  2000  9 2000
  2001 10 2001
  </code></pre>