Add field to exclude classes

[FLacombe]

Dear maintainers,

Recent releases of osrm offer the ability to exclude classes when asking for a route with exclude parameter.
It would be useful to add a simple text field beside start/end/step on frontend to manually specify a string that would be sent to route service in appropriate parameter.

A single textfield is the simplest way of doing so. A config file can give a list of possible classes and use a multi-select input to be more precise.

How do you feel about this?