从给定的电子邮件列表中找到最常见的主题行，并返回该主题行及其出现次数。
email_list = ["Subject: Discount code", "Body: Get 20% off your next purchase!", "Subject: Important Announcement", "Body: Please read the attachment for more information.", "Subject: Monthly Newsletter", "Body: Here are some updates from our company."]
"Subject: Monthly Newsletter" 出现了 1 次，"Subject: Discount code" 和 "Subject: Important Announcement" 各出现了 1 次。