gutenbergr 0.4.0

• gutenbergr now caches works downloaded with gutenberg_download(). They are saved in a temporary directory by default, but they can be configured to persist in your OS-specific application cache directory across sessions (@jrdnbradford, #112, #123).
• A new family of gutenberg_cache_* functions are now available. These allow users to list and delete items in their cache, as well as update their cache type (@jrdnbradford, #112, #123).
• The package index reference in the pkgdown site now separates functions and data by type (@jrdnbradford, #114).
• Jordan Bradford (@jrdnbradford) is now the primary maintainer of this package. Thanks for taking the lead, Jordan! (#95).