Runtime Observable Settings

[psuzn]

This adds support for the wrapper around the normal setting (#155 ) to make it runtime observable.

@russhwolf I have named it runtime-observable but we can rename it if there is a better name.

I also have added the checkObservable flag on the converter extension function, so one can just use the no-arg initializer and use the converter to make it observable. In some platforms, the instance returned by no-arg is already observable; in that case, the converter doesn't add any wrapper.