epubr 0.6.0 release

• Added count_words helper function.
• Improved word count accuracy in epub. Now also splitting words on new line characters rather than only on spaces. Now also ignoring vector elements in the split result that are most likely to not be words, such as stranded pieces of punctuation.
• Added epub_recombine for breaking apart and recombining text sections into new data frame rows using alternative breaks based on a regular expression pattern.
• Added epub_sift function for filtering out small text sections based on low word or character count. This function can also be used directly inside calls to epub_recombine through an argument list.
• Added epub_reorder for reordering a specified (by index) subset of text section data frame rows according to a text parsing function.
• Refactored code to remove purrr dependency.
• Added unit tests.
• Updated function documentation, readme and vignette.