add MAILTO to cron groups

Add mailto:

    cron = Plan('cron', mailto=['someone@zhihu.com', 'someone2@zhihu.com'])

it will generate code below:

    # Begin Plan generated jobs for: zhihu
    # jobs will send mail to users below
    MAILTO=someone@zhihu.com,someone2@zhihu.com

Only support named cron.