This function calls the command ebook-convert in Calibre
(https://calibre-ebook.com) to convert e-books.
calibre(input, output, options = "")The output filename.
The input filename.
The output filename or extension (if only an extension is
provided, the output filename will be the input filename with its extension
replaced by output; for example, calibre('foo.epub', 'mobi')
generates foo.mobi).
A character vector of additional options to be passed to
ebook-convert.