Allow application to configure initial window size, etc

[olix0r]

gRPC clients, for instance, set an initial window of ~1MB. We should have a way to configure window sizes on clients and servers (i.e. esp prior to handshake).

[seanmonstar]

Both the Client and the Server now have builders that can currently configure:

• initial_window_size
• max_frame_size

[seanmonstar]

A client can now disable push promises.

I think that perhaps we can close this, and any individual settings required can be separate issues, since they aren't trivial things to add.