config: Introduce config_option method

config_option to retrieves a configuration option given a key. If there
is no pandoc configuration present, or if the key is not present in that
section, config_option will return its default parameter.
config: support output format configuration option

By default, pandoc will use the html5 output format. A user can can
change the output format in _config.yml by specifying the format option
of the pandoc section, e.g.,

    pandoc:
      format: html
config: update README