FEATURE: allow publishing the same event in several channels

[inigomarquinez]

Detailed Description

A user suggested in this slack thread that it would be interesting to have the possibility to publish the same event in several slack channels.

Context

This feature would be useful so for example, Bank Holidays can be published both in a Company internal channel for the employees and in a slack channel shared with clients, so they are automatically informed about the Company Bank Holidays.

Possible Implementation

Just support an array of webhooks in the env variables.