feat(olHelpers): preserve custom layerConfig properties
feat(olHelpers): preserve custom layerConfig properties

Copy over eventual custom properties on the layerConfig passed to the layer directive
and add it to the native Openlayers layer. This allows to later get those properties
again via `layer.get('propName')` or `layer.getProperties()` which might be helpful
for identifying those layers.

Initialize layers with custom properties to bind custom information on
ol.layer objects.

tombatossals/angular-openlayers-directive,polesapart/angular-openlayers-directive,ageblade/angular-openlayers-directive,gaf-ag/angular-openlayers-directive