Merge Mermaid Functionality for custom date/timeline axis


Per this github thread:

Can we please add this functionality to stackedit? I write my project notes in stackedit and generate gantt timelines, but internal constituents want actual dates, so I have to manually screenshot the gantt chart, put it in powerpoint/photoshop, and add dates manually.

This feature would save me ~10 minutes a week for each gantt chart I generate (~4 or 5).