title: newtonsoft json serialize timespan format
body: <p>Is it possible to specify custom format for <code>TimeSpan</code> serialization?
  Using <code>Newtonsoft.Json</code>.</p>
  
  <p>I would like to have serialized string in format HH:mm, so for example:</p>
  
  <p><code>TimeSpan.FromHours(5)</code> -> // "+05:00"</p>
  
  <p><code>TimeSpan.FromHours(-5)</code> -> // "-05:00"</p>
  
  <p>Thanks!</p>