title: Read data from multiple CSV files into single ffdf object
body: <p>Is it possíble to load at once data from several files into a ff data frame (ffdf)?
  Lets say I have</p>
  
  <pre><code>big_file_part1.csv
  big_file_part2.csv
  big_file_part3.csv
  </code></pre>
  
  <p>I know I could load each csv file to a separate ffdf object and then ffdfrbind.fill them together. 
  But that seems like an inefficient way, loading stuff twice. Is there more direct way?</p>