Removing else statements of (default) arguments

All default values are already defined with [`Jekyll::DEFAULTS`](https://github.com/mojombo/jekyll/blob/master/lib/jekyll.rb#L53). Or just using a `_config.yml` to define more arguments and options, which are not provided by grunt-jekyll. There is no need to reference to `_config.yml`, because Jekyll will check this file in grunt-jekyll's root directory by default.