Add basic implementation for selecting between route geometries

Not a great implementation, but it works. If no route geometry choice is
made it defaults to the default "route_geometry". If
"alternative_geometries" is specified it uses the first one of the
alternatives (because as far as I can tell there is only ever one
alternative? I supposed at some point I could extend if needed, perhaps
use a tuple argument: {"alternative_geomtries", 1}).