version to 0.20.0

* Split `Model::Configuration` into mixin (#96)
* Split `Model::Fields` into mixin (#102)
* Split `Model::Associations` into mixin (#105)
* Split `Model::Persistence` into mixin (#107)
* Don't use `NsOptions` for model configuration (#113)
* Have association writers return the value they wrote (#114)
* Have `MR::Model` use new mixins logic (#115, #95)
* Remove model association validation (#116)

/cc @kellyredding