Add options to force a full rebuild
First failing feature for incremental regeneration
Add exclusion of features tagged with "@wip"
Mark the existing features as @wip
Document a couple of other scenarios
Store file metadata for a Jekyll site

Introduce a class Jekyll::SiteMetaData. It's a semi-generic storage
thing based using PStore in the Ruby standard library to store file
modification times for the purposes of incremental regeneration
Verify that our new metadata file is always filtered