Add a feature gate for service

[hgzimmerman]

Fixes #199

Allows specifying if the RouterService is compiled - which stdweb support is contingent upon.

This should allow the library to be used outside of a web environment as it breaks the dependency on anything relying on emscripten.

[hgzimmerman]

This is a breaking change for anyone who uses the library without any features at the moment.
They should be instructed to add the feature "service" to their yew_router dependency.

[hgzimmerman]

As of now, another breaking change has been introduced.

Option is no longer allowed in Switches, now either use the crate-provided Permissive or create another variant instead.