title: Mocha: get stacktrace when running mocha programmatically
body: <p>I am running tests using mocha programmatically and able to run the tests, get full title using following code but I couldn't find any way to get complete stack trace that is being printed on the commandline. How do I get the complete stack trace using mocha as library.</p>
  
  <pre><code>mocha.run(function(failures){
      if (failures) {
          console.log('it failed');
      } else {
          console.log('it passed');
      }
      process.on('exit', function () {
          process.exit(failures);
      });
  }).on('fail', function(test){
      title =  test.fullTitle();
  }).on('pass', function(test){
      title =  test.fullTitle();
  });
  </code></pre>