Add -m migrations flag and refactoring

I thought about your [comment](https://github.com/frenesim/schema_to_scaffold/pull/11#issuecomment-48197351) that you use migrations, and decided that it should be a flag, with the default set to `--no-migrations` and a discussion in the Readme.

While at it, I also did some refactoring, to try to improve the CodeClimate score.

And, I revved the version number, as I hope you'll push the gem with these changes.