g-map-route expose calculated distance from service

[thoresuenert]

When using g-map-route to display a route between two lat lng destinations.
If you want to show human readable information about distance and duration there is no chance to get that info.

We introduce a callback called: onDistanceUpdate

• the first leg of direction object will be exposed to the handler

[coveralls]

Coverage remained the same at 100.0% when pulling ff98bd1 on thoresuenert:pu/route-expose-calculated-distance-from-service into 63b0fd0 on asennikov:master.

[thoresuenert]

• add docs
• fix travis issue