Implement IgnoreRequired

I just noticed `Hashie::Extensions::IgnoreUndeclared` and thought that a similar mixin could be useful to implement a more maintainable `Hashie::Bash` like I proposed in https://github.com/intridea/hashie/issues/229. 

This PR implements `Hashie::Extensions::IgnoreRequired` which is similar to IgnoreUndeclared and could be an important building-block of a future `Hashie::Bash` implementation. This mixin disables required constraints, which was a main feature of the `Hashie::Bash`, but it does not handle nested structures or have a fluent API for assignment like the original proposal in #229.