Handle subscriber URIs (#214)
Handle subscriber URIs (#214)

Flows support a TargetURI as an escape hatch instead of a Target
resource. This value is passed through to the Subscription's subscriber.
Buses will now accept an http:// or https:// based URI as the
subscriber.

It is generally recomended to avoid using a URI unless nessesary. A DNS
host name is preferred as it will be more compatible with the future
event delivery protocol, which will detect/negotiate the underlying
transport.

Refs #189
knative/eventing,knative/eventing