Syntax: #![attribute_name] (like in Rust)

After reading https://ozkriff.github.io/2017-12-01--devlog.html where @ozkriff said that his configs (https://raw.githubusercontent.com/ozkriff/zemeroth_assets/master/objects.ron) aren't looking very nice because of how RON handles newtypes, I thought about adding attributes which allow to configure RON for your needs. Not only would this allow for toggles like #![unwrap_newtypes], but it could also be used to deal with new features. That way, we can just enforce that you need #![feature_name] to enable a RON feature, such that a parser without support for that feature can return a simple and understandable error.