Allow additional keyword arguments to collection call.

[vizcacha]

I have a use case where I need to send additional parameters for the collection request. This is achieved with a simple modification to the collection method.

I'm not sure if you are looking for help, but I figured I'd send my changes your way just in case.

[lcosmin]

Any help is welcome 😁 !

In this case, I'd rather add the parameters explicitly (something like: def collection(self, user_name, brief=True, version=True, ...) ) instead of using **kwargs, so that people who aren't familiar with the BGG API can see the parameters in their IDEs.

I'll create an issue and try to implement it over the next days; feel free to send another pull request in the mean while, if you want to, but use the develop branch, not master.