put geom_event() function in {epikit}

[nsbatra]

Idea for wrapper package, or for Epi R Handbook section

Make it easy to add notable events onto an ggplot epicurve. So for example, to add vertical lines for events like "vaccination campaign begins" or "social distancing order".

Perhaps a geom_event() function that accepts:

• a date
• string to display,
• string wrap length,
• text color
• height (% of plot height)
• line type (e.g. dashed)

[aspina7]

cross-reference with #279