Consistent definition of options and their defaults

[krlmlr]

Ideas borrowed from renv, CC @kevinushey.

Bundles all options under ?pillar_options.

This proposal uses a tiny bit of metaprogramming. It allows accessing options via pillar_options$bold() and setting them via pillar_options$bold(value) . Local options are supported too. In addition, there are internal accessors of the form get_pillar_option_bold(), set_pillar_option_bold() and local_pillar_option_bold().

The infastructure file aaa-options.R is package-agnostic, the options.R file contains only a little boilerplate code and the definition of the option getters; to use e.g. in tibble I'd replace pillar with tibble in the file and tweak the tibble_options object.

Tests were created and edited using the multi-cursor feature of RStudio.