Have an additional check for commit message formatting

[brendanheywood]

This is on par with the moodle core requirement that every commit reference an MDL tracker. 90% of moodle plugin repos will use github so referencing a github issue via the #123 syntax in the commit makes the most sense but we'd want that to be configurable, probably just a regex.

• ignore merge commits and similar
• have some sort of configurable regex or template in the travis yaml