Add support for polyline POIs just like in the good ol'
Google Maps days. See #883.
The format after this commit is:
{ id, x, y, z, text, color, polyline/polygon: [{ x, y, z }, ...] }
Optional properties:
- common ones like "icon" and "hovertext"
- "strokeColor" (string),
- "fill" (boolean)
- "weight" (integer)
Docs not included.
Largely based on PR #1536 by @jsmienk.
Closes#1456, closes#1536, closes#1643.
