User Streams account specific parameters (with=following and with=user)

Hello!

Firstly thanks for the awesome library! 

So I kinda needed the "with=following/with=user" functionality when creating UserStream. These parameters are mentioned here:

https://dev.twitter.com/docs/streaming-apis/streams/user

"Data from accounts the user follows

The with parameter controls the types of messages received. The default for User Streams is with=followings, which gives data about the user and about the user’s followings (accounts which the authenticated user follows). The setting with=user sends only events about the user, and not events about their followings."

Apparently I couldn't locate any relevant configuration, so I extend the library. Its working fine in my deployed application. Hope you find it helpful :)

Thanks