Add id to GeoJson

[hadrienlemaire]

Hi,

In order to change the color of a specific trace, I've added few lines to add an id attribute to the <path> tag of each GeoJson object. Basically, if an id key is present inside the properties object of the GeoJson, its content will be used to create the id attribute on the map. Otherwise, nothing is done. It works just like the popup text and link.