MultiGeometry Support

[sergej-singer]

Related to #1
The usual organisation of KML with MultiGeometry looks like this:

Folder
  Placemark 1
    MultiGeometry
      LineString/Polygon/Point 1
      LineString/Polygon/Point 2
      ...
      LineString/Polygon/Point n
  Placemark 2
    LineString/Polygon/Point 

MultiGeometries can also contain MultiGeometries, but I didn't implement this yet.

MultiGeometries are implemented as Lists of LineStrings, Polygons or Points. There's also a check for cases when MultiGeometry contains more than one type of Geometry, otherwise it couldn't rendered it with jinja functions.

For every Jinja Function I've added "multi" function, which can process such lists with geometries.

I've tried to test every case I could imagine, looks like it's working.