Add Support for More Content File Formats

     Currently, only Markdown, including its GitLab-flavored dialect, is supported. It'd be nice if support could get added for:

• At least some of the formats currently supported by Pandoc as listed on the project's home page, namely including:
  • HTML5 (Produce its XML serialization in 'XHTML 5' automatically on export by default.)
  • EPUB
  • \LaTeX
  • Microsoft Office Open XML, primarily Word '.docx.'
  • OpenDocument format, primarily OpenDocument Text ('.odt.')
  • MediaWiki wikitext
  • PDF (Portable Document Format)
• Mobipocket ('.mobi') (Despite its deprecation; at least one known possible user might appreciate being able to export it.)
• Rich Text Format (RTF)

Additionally or alternatively, Calibre could also help serve to convert between supported formats.