Add hooks for operations

[ml8]

Why

To expose telemetry.

For tutoring, for example, it is required to have a record of activity. This can be parsed from logs, but it would be better to have explicit hooks (e.g., PUT/POST requests). Imagining using this API for Sheets, for example.

Expected Behavior

Via flags or a config file, let users specify some hooks for various operations. When an operation occurs, that hook is notified. At the very least, support some sort of web hooks.

[ml8]

These hooks could start as flags if desired, and would be inserted in the following places:

• https://github.com/matthewlang/slack-queue/blob/a42e50495f57b21a7db5d584b5680a6c1255b9c9/pkg/service/put_command.go#L62
• https://github.com/matthewlang/slack-queue/blob/a42e50495f57b21a7db5d584b5680a6c1255b9c9/pkg/service/take_action.go#L92
• https://github.com/matthewlang/slack-queue/blob/a42e50495f57b21a7db5d584b5680a6c1255b9c9/pkg/service/take_command.go#L75
• https://github.com/matthewlang/slack-queue/blob/a42e50495f57b21a7db5d584b5680a6c1255b9c9/pkg/service/remove_action.go#L58
• https://github.com/matthewlang/slack-queue/blob/a42e50495f57b21a7db5d584b5680a6c1255b9c9/pkg/service/move_action.go#L63

The simplest thing would be to have flags that are templates for HTTP post requests.