Support [<BasePath>]

[Smaug123]

E.g. from their docs:

[BasePath("v1")]
public interface ISomeApi
{
    [Get("first")]
    Task<FirstResponse> GetFirstAsync();

    [Get("{url}")]
    Task GetSecondAsync([Path] string url);
}

Remember, this needs to respect any /absolute/paths in the member annotations: if we pass /foo to GetSecondAsync, the result is that we skip the v1 from the BasePath.

Also from the docs:

If it's specified both ways, then the [BaseAddress(...)] attribute is ignored. This means that you can have a default Base Address specified on the interface, but this can be overridden when actually instantiating it using RestClient.

[Smaug123]

Oops, I implemented BaseAddress and not BasePath in #44.