Refactoring to make fods (ODF Flat XML) file support easier

[sonota88]

Summary

By this PR, users will be able to read fods files by creating a class that extends the LibreOffice or OpenOffice class as follows.

module Roo
  class LibreOfficeFlatXml < LibreOffice
    def supported_extension
      '.fods'
    end

    def open_oo_file(options)
      roo_content_xml_path = ::File.join(@tmpdir, 'roo_content.xml')
      FileUtils.cp @filename, roo_content_xml_path
    end

    # Unlike content.xml in ods files, fods files contain line breaks,
    # so text nodes need to be excluded.
    def cell_elements(table_element)
      table_element.children
        .reject { |child| child.is_a? Nokogiri::XML::Text }
    end
  end
end

Originally I wanted to support reading ODF Flat XML, but limited the changes to the bare minimum to avoid big PR.
As you can see, these changes are simple refactorings, so these changes obviously don't break any tests.

Other Information

Nothing in particular.

[coveralls]

Coverage increased (+0.01%) to 94.348% when pulling 00773dc on sonota88:refactor_for_fods into d416f15 on roo-rb:master.

[simonoff]

@sonota88 I think will be better to create also a Libreoffice class too.

[paddor]

What's the status here? I'd highly appreciate FODS support.