Support Slack-style webhooks

[Techman]

Supporting Slack-style webhooks would allow Notifico to be used in a more generic interface that can support many services that provide webhooks, such as server monitoring services.

Another potential avenue would be Discord-style webhooks, but since Discord has a Slack webhook compatibility layer, I think Slack should have more priority.

[TkTech]

There will definitely be limits to what we can reasonably do with this as a v1. Slack webhooks can be very complex, and even interactive.