Support New Incoming Webhook specs

[nandenjin]

Since Slack Incoming Webhook renewed, now we cannot set username and icons of messages. Because of that, we cannot get email addresses of sender from message. We have to make them be included to message body.

https://api.slack.com/messaging/webhooks

You cannot override the default channel (chosen by the user who installed your app), username, or icon when you're using Incoming Webhooks to post messages. Instead, these values will always inherit from the associated Slack app configuration.