epubr 0.4.0 release

• Enhanced function documentation details.
• Added epub_meta for strictly parsing EPUB metadata without reading the full file contents.
• When working with a vector of EPUB files, functions now clean up each unzipped archive temp directory with unlink immediately after use, rather than after all files are read into memory or by overwriting files in a single temp directory.
• Added initial introduction vignette content.
• Minor function refactors.
• Minor bug fixes.
• Added unit tests.