epubr 0.4.1 release

• Improved handling of errors and better messages.
• More robust handling of title field when missing, redundant or requiring remapping/renaming. All outputs of epub now include a title as well as data field, even if the e-book does not have a metadata field named title.
• Minor improvements to e-book section handling.
• Added epub_head function for previewing the opening text of each e-book section.
• Removed R version from Depends field of DESCRIPTION. Package Imports that necessitated a higher R version were previously removed.
• Minor fixes.
• Updated documentation, vignette, unit tests.