Group fixes into commits

[jackfirth]

In https://github.com/racket/drracket/pull/611, it was suggested that it would be helpful if Resyntax-generated pull requests could better summarize themselves. My ideal approach to this would be for Resyntax to group fixes into commits, where each commit contains all of the fixes for a single rule. That way the rule's description can be used in the commit message. The pull request description can be the summary stats that resyntax fix currently prints in the terminal.