Update dependencies and simplify inputs

This release includes the following:

• Update @actions/github to 4.0.0 and @actions/core to 1.2.4. This includes updating http-client to 1.0.8, resolving this security vulnerability: actions/http-client#27
• Make the GITHUB_TOKEN input optional

Add support for customizing the notification message

This release includes an enhancement to customize the notification message.

Automatically add @ to the recipient list

This release includes a minor improvement. Prior to this release, if a user specified a recipient without prefacing it with @ then the individual or team would not be notified. After this release, the @ is automatically added if missing. This makes the action easier to use.

Initial release

This is the initial release of the issue-label-notification-action action. This action notifies individuals or teams when specific labels are added to an issue.