external_figure.md

``` diagram { img:src="external_figure.svg" figcaption="foo"
      .my_class_a .my_class_b  #my_id
      img:alt="a circle" figure:myattr=foo img:width=10
      svg:class="my_svg_class" svg:title="A title to SVG" svg:width=17
    }
circle 10
```