add: send to a list of mailboxes

[lzjwlt]

Send to a list of mailboxes
config like this:
"notifications":{
"mail":{
"smtpHost":"xxx.com",
"port":25,
"username":"xx@xxx.com",
"password":"xxxxxxxx",
"from":"xx@xx.com",
"to":["a@xx.com", "b@bbb.com", "c@ccc.com"]
}
},

[rmorrison61]

This looks great, you may want to update the sample_config.json file as well as the README.md and any other documentation related to the configuration. If other users would use that documentation and this PR was merged it wouldn't work for them.

[BobCashStory]

it woul cause breaking change for all using mail, could you do a support for old and new method ?