Support drawing graphs in the test/examples directory

[skieffer]

I know that elkjs does not officially support the drawing of graphs (just layout), but I wondered if you would like to add a simple script that allows us to see the graphs in the test/examples directory.

I have found this very useful when working on ELK. My workflow is:

1. Make changes in ELK.
2. Rebuild elkjs in a sibling directory.
3. Reload a page from elkjs/test/examples in the browser.
4. Repeat.

I've written SimpleGraphDrawer.js which draws ELK graphs given in JSON format as SVG.
Furthermore, it handles all of the new JSON coordinate modes proposed in eclipse-elk/elk#1071 .

This PR includes four files:

• SimpleGraphDrawer.js: The drawing class (and a supporting SvgDrawing class).
• test-browser-draw.html: Same as the existing test-browser.html, except that now we draw the graph.
• test-browser-draw2.html: Like test-browser-draw.html but with a more interesting graph (namely, the one from here ).
• test-browser-draw-all-modes.html: Draws a single graph 6 times, with all possible combinations of the new JSON coordinate modes.