docs: def check_dependencies():
      """Check external dependecies
      Return a tuple with the available generators.
      """
      available = []
      try:
          shell('ebook-convert')
          available.append('calibre')
      except OSError:
          pass
      try:
          shell('pandoc --help')
          available.append('pandoc')
      except OSError:
          pass
      if not available:
          sys.exit(error('No generator found, you cannot use md2ebook.'))
      check_dependency_epubcheck()
      return available