title: bind dataframes with names of dataframes as an additional id column
body: <p>I have 3 data frames in R 
  lets say:</p>
  
  <pre><code>df1 &lt;- data.frame(x = 1:5, y = 1:5)
  df2 &lt;- data.frame(x = 1:6, y = 1:6)
  df3 &lt;- data.frame(x = 1:4, y = 1:4)
  </code></pre>
  
  <p>and I would like to merge them together to create a new data frame that would have as first column the name of the data frame for each row of that data frame.
  I would look like this:</p>
  
  <pre><code>   df x y
  1  df1 1 1
  2  df1 2 2
  3  df1 3 3
  4  df1 4 4
  5  df1 5 5
  6  df2 1 1
  7  df2 2 2
  8  df2 3 3
  9  df2 4 4
  10 df2 5 5
  11 df2 6 6
  12 df3 1 1
  13 df3 2 2
  14 df3 3 3
  15 df3 4 4
  </code></pre>
  
  <p>Any ideas?
  Thank you in advance.</p>