Provide middleware point to handle *cljs-warnings*

* Add in a default warning middleware with that same behavior as the current warning handler.
* Centralize the default warning behavior, make side-effects from each invocation of warning controllable through middleware.
* fix no-warn macro, make sure undeclared-ns-form is passed to warning for the right forms
* Make analyzer default-warning-handler respect *cljs-warnings*
* Fix default-warning-handler implementation and all the callers
* Change cljs.analyzer/confirm-var-exists: Don't call warning unless :undeclared-var warning is set, AND the var hasn't been def'd
* Add some basic all-warn no-warn analyzer tests