title: Subtract two strings from each other
body: <p>I have the following input </p>
  
  <pre><code>#mydata
  
  ID  variable1  variable2
  1    a,b,c,d      c,a 
  2    g,f,h        h
  3    p,l,m,n,c    c,l
  </code></pre>
  
  <p>I wish to subtract the strings of varible2 from variable1 and I'd like to have the following output?</p>
  
  <pre><code>#Output
  ID  Output 
  1    b,d      
  2    g,f        
  3    p,m,n    
  </code></pre>
  
  <p>#dput</p>
  
  <pre><code>structure(list(ID = 1:3, variable1 = structure(1:3, .Label = c("a,b,c,d", 
  "g,f,h", "p,l,m,n,c"), class = "factor"), variable2 = structure(c(1L, 
   3L, 2L), .Label = c("c,a", "c,l", "h"), class = "factor")), .Names =    c("ID", 
   "variable1", "variable2"), class = "data.frame", row.names = c(NA, 
  -3L))
  </code></pre>