title: Formatting negative numbers with brackets
body: <p>I would like to format my negative numbers in "Accounting" format, i.e. with brackets.
  For example, I would like to format -1000000 as (1,000,000).</p>
  
  <p>I know the way of introducing thousands-separator:</p>
  
  <pre><code> prettyNum(-1000000, big.mark=",",scientific=F)
  </code></pre>
  
  <p>However, I am not sure how to introduce the brackets. I would like to be able to apply the formatting to a whole vector, but I would want only the negative numbers to be affected. Not that after introducing the thousands separator, the vector of numbers is now a <strong>characater vector</strong>, example:</p>
  
  <pre><code> "-50,000" "50,000"  "-50,000" "-49,979" "-48,778" "-45,279" "-41,321"
  </code></pre>
  
  <p>Any ideas? Thanks.</p>