Announce a message from pull requests into Slack

I have forgotten to announce too many times at work. This workflow involved moving from a pull request and writing out the message myself into a channel. This made me think "shouldn't think be done entirely from a PR?".

By simply adding announcement="<message>" to a pull request, the given message should be placed into a channel for the provided webhook.

This is built as a GitHub App and will require installation as such.

The application takes various environment variables which are used for configuration.

• ANNOUNCERD_GH_APP_ID is the GitHub App Application ID (required).
• ANNOUNCERD_GH_APP_KEY_FILE is the path to the .pem key file for the GitHub App (required).
• ANNOUNCERD_SLACK_WEBHOOK is the "Incoming Webhook" for Slack, configured to the channel you want to send announcements too (required).
• ANNOUNCERD_HOST is the address which announcerd binds to, by default this is localhost.
• ANNOUNCERD_PORT is the port which announcerd binds to, by default this is 6000.