add optional tags in the subject

I'd like to send additional tags in the subject of the mail, for instance with the name of the applicaiton or process reporting it.

With this patch, it is possible to do it as follows:

~~~javascript
winston.loggers.add('logger', {
  email: {
    from   : 'xxx',
    to     : 'xxx',
    service: 'Gmail',
    auth   : { user: 'xxx', pass: 'xxx'},
    tags   : ['foo', 'bar'] //optional tags for the subject
  }
  // other transports
});
~~~

Then the subject of the mail will loo like

~~~
[foo] [bar] [debug] xxxx
~~~

thanks!