add optional -elk in graph syntax regex

[zirconium-n]

closes #150

[bpruitt-goddard]

Thanks for the contribution! I've got a couple questions:

1. Where do you see that this is the way to leverage the Elk renderer? Looking at the docs, I'm seeing that it is a config setting, not something in the title.
2. Technically, this allows for the incorrect graph-elk to be defined, but that probably isn't much of a big deal.
3. Can a test file be added to confirm that the highlighting works? Probably just copy the graph-flowchart.test.mermaid file and tweak to the new syntax. Or add a second graph to that test file.