Support `field.categorical` and `field.categoriesOrdered`

[peterdesmet]

CHANGELOG: https://datapackage.org/overview/changelog/#schemacategories-new and https://datapackage.org/overview/changelog/#schemacategoriesordered-new

• Implement with interlacer package and add interlacer support via read_interlaced_resource() #213
• Update tests
• Update documentation
• How do we support users in allowing constraints.enum in still being interpreted as factors? It was never part of the spec, but it has been a feature of frictionless-r

[khusmann]

Hi Peter, just wanted to share some of my progress in this area to get your feedback on the direction I'm going. I've made a vignette to explain... please let me know what you think, when you have the chance!