Slack integration

It would be useful to have Llama post to a Slack channel (with a [Slack Webhook](https://api.slack.com/incoming-webhooks)) to announce when an instance has been terminated.
1. The Slack webhook would be an option in [`Llamafile`](https://github.com/hassy/llama-cli/blob/master/Llamafile.json), e.g. `slackWebhook`
2. The [lambda definition](https://github.com/hassy/llama-cli/blob/master/lambda/index.js) would use that (if defined) to post a message.

Start with Llamafile.json and lambda/index.js, the two files named in the issue, and trace how termination events and configuration are handled. Done means a configured Slack incoming webhook receives an announcement when an instance terminates, while the integration remains optional when no webhook is defined.

Written by the indexing model from the issue text.