Move to a fall-through config hash.

This is somewhat easier to understand than the current solution,
but does have some downsides: namely it doesn't work for things like
Pry::Hooks.

It is however the only way I've thought of to let setting
Pry.config.color = work in addition to _pry.config.color =.

For now I've added a hack so that hooks work as they did before,
in future we should work out how to fix them so that only the data
for hooks is stored in the config object, not the hooks object itself.

lib/pry/config/behavior.rb

@@ -19,11 +19,7 @@ module Pry::Config::Behavior
   end
 
   def initialize(default = Pry.config)
-    if default
+    @default = default
-      @default = default.dup
-      @default.default_for(self)
-    end
-    @default_for = nil
     @lookup = {}
   end
 
@@ -56,7 +52,10 @@ module Pry::Config::Behavior
       self[key]
     elsif @default.respond_to?(name)
       value = @default.public_send(name, *args, &block)
-      self[key] = _dup(value)
+      # FIXME: refactor Pry::Hook so that it stores config on the config object,
+      # so that we can use the normal strategy.
+      self[key] = value.dup if key == 'hooks'
+      value
     else
       nil
     end
@@ -94,14 +93,6 @@ module Pry::Config::Behavior
     @lookup.delete(key.to_s)
   end
 
-  def default_for(other)
-    if @default_for
-      raise RuntimeError, "self is already the default for %s" % _clip_inspect(@default_for)
-    else
-      @default_for = other
-    end
-  end
-
   def keys
     @lookup.keys
   end

spec/config_spec.rb

@@ -21,12 +21,6 @@ describe Pry::Config do
       local.foo.should == 2
     end
 
-    it "duplicates a copy on read from the parent" do
-      ukraine = "i love"
-      local = Pry::Config.new Pry::Config.from_hash(home: ukraine)
-      local.home.equal?(ukraine).should == false
-    end
-
     it "traverses through a chain of parents" do
       root = Pry::Config.from_hash({foo: 21})
       local1 = Pry::Config.new(root)