Make watch expressions fully global

At the moment Pry::Hooks are not local to each instance, so the hook
was only being added once. This caused problems when you opened two
binding.pry's in one program, as the second one's watch expressions
appeared to be ignored.

lib/pry/commands/watch_expression.rb

@@ -38,7 +38,7 @@ class Pry
     private
 
     def expressions
-      state.expressions ||= []
+      Pry.config.watch_expressions ||= []
     end
 
     def delete(index)
@@ -66,7 +66,7 @@ class Pry
       end
     end
 
-    def eval_and_print_changed
+    def eval_and_print_changed(output)
       expressions.each do |expr|
         expr.eval!
         if expr.changed?
@@ -82,9 +82,9 @@ class Pry
 
     def add_hook
       hook = [:after_eval, :watch_expression]
-      unless Pry.config.hooks.hook_exists?(*hook)
-        _pry_.hooks.add_hook(*hook) do
-          eval_and_print_changed
+      unless Pry.hooks.hook_exists?(*hook)
+        Pry.hooks.add_hook(*hook) do |_, _pry_|
+          eval_and_print_changed _pry_.output
         end
       end
     end

spec/commands/watch_expression_spec.rb

@@ -64,6 +64,30 @@ describe "watch expression" do
     end
   end
 
+  it "continues to work if you start a second pry instance" do
+    ReplTester.start do
+      input 'a = 1'
+      output '=> 1'
+
+      input 'watch a'
+      output "Watching a\nwatch: a => 1"
+
+      input "a = 2"
+      output "watch: a => 2\n=> 2"
+    end
+
+    ReplTester.start do
+      input 'b = 1'
+      output '=> 1'
+
+      input 'watch b'
+      output "Watching b\nwatch: b => 1"
+
+      input "b = 2"
+      output "watch: b => 2\n=> 2"
+    end
+  end
+
   describe "deleting expressions" do
     before do
       eval 'watch :keeper'