Add a github formatter

[thompson-tomo]

I am interested in using vint and while looking at features I come across the formatter config options. When I look at the options via source code, I notice there is no github formatter which is useful for anyone wanting to use it in a ci environment.

See https://docs.github.com/en/actions/reference/workflows-and-actions/workflow-commands#setting-a-warning-message for an example on the format the log messages expect.