Refactor tool into tool+package

[emicklei]

Describe the Feature

More of a refactoring request. Currently the slack-notifier is a program, not a reusable package.

Use Case

Use slack notifier from a running WebService.

Describe Ideal Solution

I would like to import slacknotifier as a package and provide (overridden) message details as parameters.

	notifier := slacknotifier.New(webhookURL)
	err := notifier.Notify(payload)

The current main.go could import that package. A Go convention is to create a cmd/slack-notifier folder where the main.go will be.

In the past, I did a similar refactoring for https://github.com/kramphub/kiya