Before this change if the value of a Node was "" it would get dropped from the json
encoding and the Node.Value field would be missing. Fix this problem by making
NodeExtern.Value a *string so that an empty string will be encoded but a nil value
will drop the field.
Refactor config into its own package. Trying to tease the config from
the server so that all of the control surfaces are exposed in the Server
for easier testing.
