gutenbergr 0.5.0

• A new gutenberg_add_sections() function is available to create forward-filled section markers (@jrdnbradford, #138)
• gutenberg_download() now adds a User-Agent string to identify itself to the Project Gutenberg mirror (@jrdnbradford, #139)
• Documentation now includes two vignettes that detail advanced usage (@jrdnbradford, #142, #143, #144)