WIP: Export pillar_options

[krlmlr]

This is a draft for an interface to pillar's options, as a list of accessors. Other options are:

• Getters and setters with a consistent syntax
• A knitr-like options object with explicit $get() and $set() methods

Rendered documentation for this PR: https://pillar.r-lib.org/options/reference/pillar_options.html .