Rephrase docstrings for directory variables
Fix docstring typo
Rename bog-readme.org to README
Rename changelog.org to NEWS
Expand headings on README startup
Prompt with bib citekeys in bog-find-citekey-bib

When a prefix argument is given to `bog-find-citekey-bib',
`bog-pdf-citekeys` was being used to generate the collection of
available citekeys instead of`bog-bib-citekeys'.
Check stage directory in renaming test
Generalize PDF functions to any file type
Clarify some docstrings