Add D2 layout engine option

[felixvanoost]

Adds support for alternate D2 layout engines (see documentation). Currently this includes both the default layout engine, dagre, and an open-source third-party alternative, ELK.

The TALA engine is not supported because it is proprietary (non-free) and closed-source.

Output with no layout specified (dagre is used by default):

curl http://localhost:8000/d2/svg/eNo9zLkRgDAMRNF8q3ADtEAvvrExBvpPkH9A8rSjkdZvuwuKjpHMrILBLFhV_3xgw6RuRvRsupp50jM06LnIy6nJbyIXXF-3Hm6y-X5cSiFx > default.svg

Output with layout=elk:

curl http://localhost:8000/d2/svg/eNo9zLkRgDAMRNF8q3ADtEAvvrExBvpPkH9A8rSjkdZvuwuKjpHMrILBLFhV_3xgw6RuRvRsupp50jM06LnIy6nJbyIXXF-3Hm6y-X5cSiFx?layout=elk > elk.svg

Closes #1557

[ggrossetie]

It looks good to me.
@felixvanoost Is it ready for review?

[felixvanoost]

@ggrossetie I made these changes while travelling and haven't had time to test them yet. If someone wants to test that the ELK layout engine works, that should be enough. Otherwise I can do it myself next week!

[f3l1x]

Looking forward. 👍

[felixvanoost]

@ggrossetie I finally had time to test this; results are in the PR description above. It's working!

[ggrossetie]

That's quite nice 😍