Allow matching routes without capturing arguments

For proxy scenarios where routes need to be parameterized for matching but not for capture support a syntax to avoid allocating anything after matching.

e.g.

```C#
// ProxyRequest is a made up method that proxies the HttpRequest to another server

endpoints.Map("/api/v1/{*path}", context =>
{
return context.ProxyRequest("http://localhost/old/" + context.Request.Path);
});

endpoints.Map("/api/v2/{*path}", context =>
{
return context.ProxyRequest("http://localhost/new/" + context.Request.Path);
});
```

cc @Tratcher