Add Topo from Tracestack

[pyrog]

Hi,

Could you add this layer recently added to OSM website ?

URL is https://tile.tracestrack.com/topo__/{z}/{x}/{y}.png?key=your_key
Tracestack repositories.

See also #145

[zod]

The free tier from Tracketrack only includes 100,000 tile views per month, but OpenCycleMap had about 1,340,000 tile views. I'm not sure if that's a good replacement.