title: Replace values in several data.table's columns that meet a condition
body: <pre><code>library(data.table)
  data = data.table("cat" = c(0,5,NA,0,0,0),
                    "horse" = c(0,4,2,1,1,3),
                    "fox" = c(2,2,NA,NA,7,0))
  </code></pre>
  
  <p>I wish to replace values of 'cat' and 'fox' that are equal to '0' or '2' with '-99'</p>
  
  <p>I can do it one at a time but how to do them both?</p>
  
  <pre><code>dat[fox == 0 | fox == 2, fox := -99]
  </code></pre>