Allow for new line before brace

Fixes #249 

I realized that because of the significance of whitespace in YAML, allowing arbitrary strings representing whitespace in a config param (e.g. " ", "   ", etc.) would be difficult or impossible; so I just went with the initial suggestion -- fitting the established pattern -- of providing named options.

So now you can choose `space` or `new_line` for the `style` option on the SpaceBeforeBrace linter.