add an option to clear console when using autoWatch:true

This is based on http://stackoverflow.com/questions/22824255/karma-clear-screen-after-running-tests.

Simply adding this in watcher.js at line 91, makes this work (at least on windows). All that is left to do is add the options to the config and pass them to watcher.

var util = require('util');
util.print("\u001b[2J\u001b[0;0H");

Is this something we want to add? I think it's useful.

Maybe we should instead support a beforeRun script that people could use to do that instead.

A workaround is to create a clear-screen-reporter that just overrides this.onRunStart and does

this.onRunStart = function() {
   console.log("\u001b[2J\u001b[0;0H");
};