Profile-dependent router values

[jcoupey]

Currently picking a routing engine happens using -r. Then several -a/-p variants can be used to match a given profile (as seen in the vehicle.profile value) to a given routing stack.

But it is not currently possible to send different profile requests to different routing engines. This is only a technical limitation: we're already creating a child instance of routing::Wrapper for each profile. The type of that wrapper is currently common (decided by -r) but we could totally have any wrapper type for each profile.

Probably this would mean introducing a syntax similar to -a car:host1 -a bike:host2 for the router, e.g. -r car:osrm -r bike:valhalla. And in order to not break backward compatibility the -r router option should default to use router for all profiles.