Send messages to Slack

[object88]

When a helm deployment is complete, send a message to a specified Slack channel, indicating success of failure.

Consider using a plugin system along the lines of https://github.com/hashicorp/go-plugin.

[object88]

Work started in #27

Discovered that Hashicorp's go-plugin is not the right fit for this usecase. We want to define what notifiers are available through helm, and as separate pods, not as a new process within a pod. The tight-coupling from go-plugin would mean that any container that had our controller would also have to have the complete set of notifiers.