Make --css, --head-html, --body-html accept filename.

          "",

          "Useful for adding extra <style> and other tags.",

          "",

          "Also a name of an HTML file can be passed.",

          "Then the contents of that file will be read in.",

          "",

          "This option can be used repeatedly to append several",

          "things to the header.") do |html|

          @head_html += html

          @head_html += maybe_file(html)

        end

        opts.on('--body-html=HTML',

          "",

          "Useful for adding extra markup to the page.",

          "",

          "Also a name of an HTML file can be passed.",

          "Then the contents of that file will be read in.",

          "",

          "This option can be used repeatedly to append several",

          "things to the body.") do |html|

          @body_html += html

          @body_html += maybe_file(html)

        end

        opts.on('--css=CSS',

          "Extra CSS rules to include to the page.",

          "",

          "Also a name of a CSS file can be passed.",

          "Then the contents of that file will be read in.",

          "",

          "This option can be used repeatedly to append multiple",

          "chunks of CSS.") do |css|

          @css += css

          @css += maybe_file(css)

        end

        opts.on('--message=HTML',

      end.flatten

    end

    # When given string is a file, returns the contents of the file.

    # Otherwise returns the string unchanged.

    def maybe_file(str)

      path = canonical(str)

      if File.exists?(path)

        Util::IO.read(path)

      else

        str

      end

    end

    # Converts relative path to full path

    #

    # Especially important for running on Windows where C:\foo\bar