Haml hash attributes spacing linter

Add a linter for checking for spaces inside hash attribute braces, analogous to the Style/SpaceInsideHashLiteralBraces cop for spaces inside ruby hash literal braces.
We use `snake_case` with lowercase letters for configuration options in `haml-lint`&mdash;I would just call this `style`. We also don't do the `SupportedStyles` idiom, instead preferring the documentation in `lib/haml_lint/linter/README.md` to list configuration options.