title: read_excel recode number as na value
body: <p>While importing data from an excel file using the <code>read_excel</code> function in the <code>readxl</code> package, I have tried specifying a number for the <code>na</code> argument but it does not recode the missing values. The data was entered with a numeric value as the missing response.</p>
  
  <p>Example data (in r rather than an excel file though):</p>
  
  <pre><code>missing &lt;- data.frame(a = c(1, 2, 3, 4),
                        b = c(99, 2, 3, 4),
                        c = c(1, 99, 3, 4))
  </code></pre>
  
  <p>If the above data were a separate excel file I tried reading it in like so:</p>
  
  <pre><code>data &lt;- read_excel("C:/.../missing.xlsx", na = 99)
  </code></pre>
  
  <p>Instead of recoding the 99's to be NA, they just get read in as numbers. I'd like to see a data frame like this without having to use a separate step to recode the data:</p>
  
  <pre><code>  a b c
  1 1 NA 1
  2 2 2 NA
  3 3 3 3
  4 4 4 4
  </code></pre>