Provide a flag to add the commit message to the slack output

[benmatselby]

With the wiki, some folks may just click save and let GitHub write the commit message. But if folks are in the habit of adding details, then it may be useful to display this.

• Provide a config option
• See if we can identify a standard message, and only show in Slack if none standard

Follows on from https://github.com/benmatselby/gollum-page-watcher-action/issues/7