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 produces more detailed HTTP logs for Prosodys built-in HTTP
server. The format is similar to that of Apache and go into Prosodys
normal logs at the `info` level.