Gantt Charts: Request Generic Marker (Similar to todayMarker)

[robertpotts]

Is your feature request related to a problem? Please describe.
I'm frustrated that I can make very complicated series of tasks, but not easily make milestones. Tasks and milestones go hand in hand on many gantt charts. There is already the core capability demonstrated with todayMarker. This would just need to be generalized.

Describe the solution you'd like
I think it would make sense to leverage the same code to make a general marker function. Something that could take in a name, date, and style. Maybe like:

marker <name>, <date>
marker "Request Submitted", 2021-08-27

With optional syntax to do styles:

marker <name>, <date>, <style>
marker "Request Submitted", 2021-08-27, stroke-width:5px,stroke:#0f0,opacity:0.5

Or even assign a variable name that could be referenced by other objects:

marker <var>, <name>, <date>, <style>
marker marker1, "Request Submitted", 2021-08-27, stroke-width:5px,stroke:#0f0,opacity:0.5

Could be used in combination like:

marker marker1, "Request Submitted", 2021-08-27, stroke-width:5px,stroke:#0f0,opacity:0.5
Create tests for renderer          :2d
Describe gantt syntax               :active, a1, after marker1, 3d

Describe alternatives you've considered

• I've looked through the code to see if I could give more inputs to todayMarker.
• I've looked to see if there's an examples that might show me how to pass HTML/CSS to the mermaid processor.
• I've searched on Google to see if there were any StackOverflow or other posts about the same topic.
• I've searched through the issues and there doesn't seem to be an issue or feature request related to this.

Additional context
Add any other context or screenshots about the feature request here.
Happy to discuss if there are questions.