Allow Group to pass on arbitrary constructor kwargs to its Connections

[bitprophet]

Triggered by a mailing list post - Group is still very early days but it can't currently generate "useful" Connections with arbitrary kwargs, such as connect_kwargs. It's either Connection-first-posarg strings, or full on, manually generated Connections (to .from_connections).

The latter may still be somewhat useful (you still get a group object to pass around more easily, and its methods still yield grouped results, etc) but it feels pretty silly and is certainly extra boilerplate (list comprehensions, Connection constructors, etc).

We should add **kwargs to Group.__init__ and pass it into the Connection constructors within, at the very least. Then do the usual stripping-out of any Group-specific kwargs as we grow them.