add property icon for map element marker

Sample:

{{#multimaps: 52.429222277955134,13.359375~ icon=Green_marker.png}}

{{#multimaps: 52.429222277955134,13.359375
|icon=Green_marker.png
}}

Change-Id: Ifa5206c6599d801a16e19e758d7f5ed668b34162