title: How to create a custom column (sortable) in DT::datatable
body: <p>Let say I have below <code>data-table</code> -</p>
  
  <pre><code>    library(DT)
       d = data.frame(
        names = rownames(mtcars),
        date = as.Date('2015-03-23') + 1:32,
        time = as.POSIXct('2015-03-23 12:00:00', tz = 'UTC') + (1:32) * 5000,
        stringsAsFactors = FALSE
      )
  
      datatable(d, filter = 'bottom', options = list(pageLength = 5))
  </code></pre>
  
  <p>Now I want to create a new column in the <code>data-table</code> by combining <code>date+time+some-other-string</code> as <code>date_time</code>. This new column <code>date_time</code> should be sortable based only on <code>date</code> part (not the <code>time</code> part nor <code>string</code> part). I also want to give different colour for <code>date</code> part and <code>time</code> part.</p>
  
  <p>Is there any way to achieve this?</p>
  
  <p>Any pointer will be highly appreciated.</p>