As a user, I want to be able to specify whether I want the directions to a restaurants by foot or by vehicle.

## Directions

The directions to a specific restaurant are currently shown only for walking routes. A second option for getting directions for driving has to be added. Both options should be available for choosing by the user. Directions should be drawn on the map according to the selected travel mode.

# Checklist
- [x] Add two buttons for selection of a travel mode
- [x] Make travelMode accessible from outside
- [x] Add toggle effect to the buttons
- [x] Decouple onclick javascript from the html