title: Fill option for fread
body: <p>Let's say I have this txt file:</p>
  
  <pre><code>"AA",3,3,3,3
  "CC","ad",2,2,2,2,2
  "ZZ",2
  "AA",3,3,3,3
  "CC","ad",2,2,2,2,2
  </code></pre>
  
  <p>With <code>read.csv</code> I can:</p>
  
  <pre><code>&gt; read.csv("linktofile.txt", fill=T, header=F)
    V1 V2 V3 V4 V5 V6 V7
  1 AA  3  3  3  3 NA NA
  2 CC ad  2  2  2  2  2
  3 ZZ  2 NA NA NA NA NA
  4 AA  3  3  3  3 NA NA
  5 CC ad  2  2  2  2  2
  </code></pre>
  
  <p>However <code>fread</code> gives</p>
  
  <pre><code>&gt; library(data.table)
  
  &gt; fread("linktofile.txt")
     V1 V2 V3 V4 V5 V6 V7
  1: CC ad  2  2  2  2  2
  </code></pre>
  
  <p>Can I get the same result with <code>fread</code>?</p>