Add dirname to Converter

It is useful for the Sass converter, as it needs to know the path
to the asset file to process directives like @import. Currently
there is no way for a converter to know where the file come from.

Now the convert method can be as follows:

    def convert
      return Sass::Engine.new(@content, load_paths: [@dirname], syntax: :scss).render
    end
Merge branch 'add-dirname-to-converter' of https://github.com/clee704/jekyll-asset-pipeline into dirname