Custom Filter Example

Write an example providing and override reducer to filter the data in a custom way. Ensure that this satisfies the ability to create custom filters. It may also be worthwhile to add a task to griddle-render to make it so custom filters can be applied on `<Griddle />` definition rather than needing to pass in a custom reducer. 

See https://github.com/GriddleGriddle/Griddle/issues/62
