adding basic implementation of authors and sections

cc @amillet89 

https://chartbeat.com/docs/configuration_variables/#groups

goal is to add `sections` and `authors` parameters so that views can be easily sliced in chartbeat by those variables. `sections` maps neatly to `category`. i'm unsure about the `authors` spec since we don't have a dedicated property per se for `properties.author` on a `.page()` call...