Implement Subscriber Black- and Whitelisting

[iwittkau]

Hi, I am using turnpike.V2 for a project where I need subscriber black- and whitelisting, especially whitelisting.

The feature is listed in the internet-draft for the WAMP protocol:
The Web Application Messaging Protocol - 14.4.1. Subscriber Black- and Whitelisting

I have a implemneted the "eligible"-option in a fork of this repo, which was one of my first go projects, hence the implementation is not very good. I think I am able to do a proper implementation now if anybody using turnpike needs it as well or if the feature is wanted by the current maintainers.