Make `Klein` introspectable.

[tomprince]

I'm working on generating some sphinx documentation from a klein application (loosely based on the autoflask directive from sphinxcontrib-httpdomain) but I need to access _url_map and _endpoints to get the relevant information.

[brighid]

The Klein.url_map and Klein.endpoints read-only properties seem to provide the introspection you're asking for: can you say more about the specific problem you're running into?