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.