Slack as an option for existing email notifications.

Now that we have Slack Notifications, I’d really like to see the email notifications also support slack. It's important for two reasons:

1. many of our team members rarely look at email but are always looking at Slack.

2. The current Slack integration shows all messages in one channel. But with a big team, that's a lot of messages going by, with only a few that are relevant to any particular person. With this feature, I could ignore the Slack channel with all the notifications, and recieve only the ones for my stories or where I am mentioned, as direct-messages.

Currently here are the email notification categories found in https://eu.litetracker.com/notification_settings

• New stories/epcis

• ownership change

• story state

• comments

• commits

• blockers

• tasks

• reviews

Each of these sends an email under certain conditions. We could keep these settings as-is. But add one pulldown/input there to indicate if I want my notifications to be send to me (1) via email, (2) via Slack, (3) both