Copy variables from parent class

When we're subclassing existing model, copy its variables. Without this
we need to explicitly define everything in child class, like for example
which protocols to use.

Fixes ytti#1901

lib/oxidized/model/model.rb

@@ -7,12 +7,18 @@ class Model
 
     class << self
       def inherited(klass)
-        klass.instance_variable_set '@cmd',     (Hash.new { |h, k| h[k] = [] })
-        klass.instance_variable_set '@cfg',     (Hash.new { |h, k| h[k] = [] })
-        klass.instance_variable_set '@procs',   (Hash.new { |h, k| h[k] = [] })
-        klass.instance_variable_set '@expect',  []
-        klass.instance_variable_set '@comment', nil
-        klass.instance_variable_set '@prompt',  nil
+        if klass.superclass == Oxidized::Model
+          klass.instance_variable_set '@cmd',     (Hash.new { |h, k| h[k] = [] })
+          klass.instance_variable_set '@cfg',     (Hash.new { |h, k| h[k] = [] })
+          klass.instance_variable_set '@procs',   (Hash.new { |h, k| h[k] = [] })
+          klass.instance_variable_set '@expect',  []
+          klass.instance_variable_set '@comment', nil
+          klass.instance_variable_set '@prompt',  nil
+        else # we're subclassing some existing model, take its variables
+          instance_variables.each do |var|
+            klass.instance_variable_set var, instance_variable_get(var)
+          end
+        end
       end
 
       def comment(str = '# ')