Slack fix

Slack handler was not properly notifying a channel as this seemed to be using an older version of the slack api maybe? I have created an update to use the slack channel as per the documentation and also have updated it to ensure names are linked.

I have added 2 additional features to the slack notifer as well which allow you to specify the name that appears when notifying the channel (this overrides the default name configured in slack for the token) and added the ability to specify an emoji icon for the notification (which also overrides the default in slack).

These two additions are helpful and make it easier to use a single token across multiple use cases for integrations.