create collapse button via options
create collapse button via options

This adds an option to modify the creation of the collapse button

ngonzalez/leaflet-routing-machine,wbg/leaflet-routing-machine,wbg/leaflet-routing-machine,ngonzalez/leaflet-routing-machine,AW-UC/leaflet-routing-machine,AW-UC/leaflet-routing-machine,rossanag/leaflet-routing-machine,AW-UC/leaflet-routing-machine,wbg/leaflet-routing-machine,martinboehm/leaflet-routing-machine,ngonzalez/leaflet-routing-machine,rossanag/leaflet-routing-machine,martinboehm/leaflet-routing-machine,martinboehm/leaflet-routing-machine,rossanag/leaflet-routing-machine