title: Use dplyr to filter out columns containing characters
body: <p>I have a large dataframe that I would like to use the excellent package <code>dplyr</code> (Wickham) which I just recently discovered. I would like to filter out columns that contain characters. Is this possible?</p>
  
  <p>For example, in the <code>flights</code> datasets within the <code>nycflights13</code> package, how could I filter out the columns that have class <code>character</code>?</p>
  
  <pre><code>library(nycflights13)
  data(flights)
  str(flights)
  Classes ‘tbl_df’, ‘tbl’ and 'data.frame':   336776 obs. of  16 variables:
   $ year     : int  2013 2013 2013 2013 2013 2013 2013 2013 2013 2013 ...
   $ month    : int  1 1 1 1 1 1 1 1 1 1 ...
   $ day      : int  1 1 1 1 1 1 1 1 1 1 ...
   $ dep_time : int  517 533 542 544 554 554 555 557 557 558 ...
   $ dep_delay: num  2 4 2 -1 -6 -4 -5 -3 -3 -2 ...
   $ arr_time : int  830 850 923 1004 812 740 913 709 838 753 ...
   $ arr_delay: num  11 20 33 -18 -25 12 19 -14 -8 8 ...
   $ carrier  : chr  "UA" "UA" "AA" "B6" ...
   $ tailnum  : chr  "N14228" "N24211" "N619AA" "N804JB" ...
   $ flight   : int  1545 1714 1141 725 461 1696 507 5708 79 301 ...
   $ origin   : chr  "EWR" "LGA" "JFK" "JFK" ...
   $ dest     : chr  "IAH" "IAH" "MIA" "BQN" ...
   $ air_time : num  227 227 160 183 116 150 158 53 140 138 ...
   $ distance : num  1400 1416 1089 1576 762 ...
   $ hour     : num  5 5 5 5 5 5 5 5 5 5 ...
   $ minute   : num  17 33 42 44 54 54 55 57 57 58 ...
  </code></pre>
  
  <p>Any ideas?</p>