Add polygon path support

[l4nos]

I would like to be able to draw a polygon or line on the map which is possible with the standard static maps API.

I'm happy to write the method into the class and do a PR if you would accept. Would rather contribute than just fork and forget.

Essentially the consuming dev would feed in a GeoJSON array of lat,long pairs which correspond to the points of the polygon. We could also allow for the insertion using WKT strings like POLYGON.

If you are happy, say the word and i'll PR.

[mastani]

Thank you for the suggestion. It would be great. Please send a PR and I'll merge it.