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.

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summary: Storage benchmark tool
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To benchmark `mod_storage_internal`:

    prosodyctl mod_benchmark_storage internal

Does a number (aprox. 10000) of reads and writes and to the selected
storage driver and tells you how long it took.