Export Button for GeoJSON / GPX

[daniel-j-h]

Tracking here in the frontend; from Project-OSRM/project-osrm.github.com#3:

We removed GPX support from osrm-backend (see #156 #53 Project-OSRM/osrm-backend#1489).

But in the current API we're already returning a GeoJSON geometry which we could either export as is or convert to GPX on the fly (Javascript) and transparently for the user.

Forcing the user to the command line or looking through the request manually is not an option.