Initial commit of mod_slack_webhooks. This provides an HTTP-based interface to and from Prosody-hosted MUCs equivalent to Slack's incoming and outgoing webhook interfaces, allowing a variety of Slack integrations to be used with a Prosody MUC.

This module allows sending a message to another MUC room.

    @other-room: hello

The message will appear in the other room as as

    <first-room/You> hello