Add support for custom icons in events overlay on timechart

[dmehra]

Custom event markers which used to work in Jut 1.0 no longer do, we should bring back their support.

When we do, add back the attached docs example, and this to events.md doc:

Example: Overlay events with custom icons, one for a bike commute event and another for a meal event

{!docs/examples/charts/events_custom_icons.juttle!}

And a screenshot.

[dmehra]

events_custom_icons.juttle.txt - Github doesn't want to attach .juttle files 😼