Add a version number footer to all Slack messages

[mfitz]

It would be nice to see which version of Slakkit is running. A footer on every Slack message would make this clear.

Appending a block of type context to the message blocks passed to the Slack API would do the trick (icon image URL will need to be added to the _version.py file). This would look something like:

{
    "type": "context",
    "elements": [
        {
            "type": "image",
            "image_url": _version.icon_url,
            "alt_text": "Slakkit icon"
        },
        {
            "type": "mrkdwn",
            "text": "Slakkit version: *{}*".format(_version.__version__)
        }
    ]
}