marker.php : Added support for coordinates as custom variables (_LAT & _LONG)

This adds support for coordinates in custom variables.
I don't want to use the "notes" field. Since Nagios3 or Icinga you can
and should use custom variables.

I chose "_LAT" and "_LONG", as these specific custom variables as they
can also be used with the geomap featuere of NagVis. So u can easily
change between NagMap and NagVis or use both of them.