Implement Discord/Slack (Webhook?) connector

[trubesv]

We need to write the kubemen.connectors.{discord,slack,webhook}.send functions in a similar fashion to the kubemen.connectors.mattermost.send, by leveraging the kubemen.models.Change object.

Optional fancyness can also be implemented to personnalise the author thanks to the kubemen.models.Character & kubemen.models.User objects.

Documentation:

• Connectors implementation
• Configuration capabilities for said connectors
  this can be extended as much as necessary but it must follow
  the config.{CONNECTOR_NAME}_{CONFIGURATION_KEY} pattern

Resources:

• Discord Webhooks
• Slack Webhooks

[ramnes]

FWIW, Mattermost and Slack used to be compatible, so the current Mattermost connector probably already works with Slack's webhook feature (although it's deprecated).