slackrenderer has different shape, layout capabilities/bugs than mainline

[agokhale]

these mmd statement cause the slack embed to error, mainline mermaid ans the vscode plugin render them ok.
A -->|standby| wait@{ shape: hourglass, label:["ru meditation"] }
A -->|event| log@{ shape: paper-tape, label:["guru meditation"]}

Also the subgraph direction rendered has alternate behavior; a little odd, perhpaps not incorrect though.

This .md demonstrates both behaviors.
https://github.com/agokhale/cantrips/blob/0033921308ff61be42d94e5d260e562c5f48fcdb/README.md