Update graph keyword regex

[kib]

Supports flowchart-elk and makes the naming or direction of a graph optional

closes #149 and #150

[bpruitt-goddard]

Thanks for the contribution!
Just a few comments:

1. I think the capture group around (-elk) is messing with the other captures. I think it needs to be something like a non-capture group ((?:-elk)). This is interferring with the capture group highlighting, resulting in the -elk getting highlighted instead of the direction:
   
2. The * after the -elk is allowing it to match multiple results, which means this (incorrectly) highlights:
   
3. While in there anyway, do you think the direction should be clarified to match the existing subgraph regex here? Since currently it allows any string to match rather than the proper directions: