Adds support for StyleMap

[alexandrupele]

https://developers.google.com/kml/documentation/kmlreference#stylemap
I am using this lib on both Android and iOS versions of my app.
The KML file that I am using contains StyleMap tags. On Android it renders well, but on iOS the Placemarks have no colors.

I've noticed that the KML Parser in the iOS lib has support for fewer tags than the one for Android, so I had to add this one myself. Maybe this will be useful for others as well..

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[alexandrupele]

I signed it!

[googlebot]

CLAs look good, thanks!

[domesticmouse]

Thanks!

[alexbartisro]

Thanks for this!

[sturza]

thanks