Replace "Mail" with display-time-mail-string in mode-line

[gongzhitaao]

trafficstars

https://github.com/iqbalansari/mu4e-alert/blob/96a293b28646f4620e257f24748becc4a06843cd/mu4e-alert.el#L294

I think it might be better to replace "Mail" with display-time-mail-string. According to the doc,

String to use as the mail indicator in display-time-string-forms. This can use the Unicode letter character if you can display it.

1. By default, its value is "Mail", but could be changed to a Unicode mail character, e.g., Font Awesome envelope icon.
2. Besides, it does not conflict with display-time-mail-icon.

[iqbalansari]

That is a great idea, I will try pushing this soon. In the meanwhile pull requests are welcome.

Thanks