title: Generate cron expression for weekly and repeat every n weeks
body: <p>I am trying to create weekly cron expression which will execute every week on selected days like Mon,Tue. Along with this I have to implement repeat every functionality with it. So that trigger would be fired after repeat every interval.</p>
  
  <p>eg. I have to execute job every Monday and alternate week in case when interval value is 2.
  When interval value is 3 I have to execute this job every Monday after 2 weeks.</p>
  
  <p>This functionality is easily achieved in case of Daily or monthly, but I am not able to find it in case of weekly.
  eg. Cron for daily and repeat every interval as 3
      0 0 12 1/3 * ? *</p>