Add vendor-kwargs to WMTS constructor.

As initially suggested in SciTools/cartopy#502...

This allows a `WebMapTileService` to use a given set of vendor-specific parameters for all requests, including the initial capabilities request.

I'd appreciate some guidance on how best to test this capability. Typically I'd add a unit test which mocks the server responses and checks the request URLs in the process, but I can't see any existing tests of that sort. Mock responses would have a number of benefits in several areas:
 - performance,
 - repeatability,
 - and because the driving use case is for specifying private API keys for restricted access services.